Fix #298: move Sentinel credentials from browser storage to backend .env

Reported by @tg12. Pre-fix, the Settings panel stored real third-party
Copernicus CDSE client_id + client_secret in browser localStorage /
sessionStorage via the privacy storage helper, and the proxy routes
required those values to come back in every tile/token request body.
Any same-origin script (XSS, malicious browser extension, dev-tools
HAR export) had read access to the credentials.

This change moves them server-side, behind the same .env-backed admin
flow every other third-party API key (OpenSky, AIS Stream, Finnhub,
Shodan, …) already uses.

Backend
-------
backend/services/api_settings.py
  * Added SENTINEL_CLIENT_ID and SENTINEL_CLIENT_SECRET entries to
    API_REGISTRY. The existing GET/PUT /api/settings/api-keys flow
    (already require_local_operator-gated, .env-backed) now manages
    them — no new route surface.

backend/routers/tools.py
  * /api/sentinel/token and /api/sentinel/tile resolve credentials via
    a new _resolve_sentinel_credentials() helper: body fields win for
    back-compat with any legacy callers, otherwise the helper reads
    SENTINEL_CLIENT_ID / SENTINEL_CLIENT_SECRET from os.environ.
  * When neither source has a value, the route returns 400 with a
    friendly pointer ("Set SENTINEL_CLIENT_ID and SENTINEL_CLIENT_SECRET
    in the API Keys panel") instead of the curt "required" message.
    The user's standing rule against hostile errors applies.
  * Function bodies only — decorator lines untouched, so this PR does
    not conflict with #303 (which adds Depends(require_local_operator)
    to the same routes).

Frontend
--------
frontend/src/lib/sentinelHub.ts — rewritten
  * Removed: getSentinelCredentials / setSentinelCredentials /
    clearSentinelCredentials / getSentinelCredentialStorageMode.
    These were the browser-storage read/write helpers; their existence
    was the bug.
  * Added: checkBackendSentinelStatus(), refreshSentinelStatus(),
    getCachedSentinelStatus(), and a kept-for-back-compat
    hasSentinelCredentials() shim. Status is sourced from
    /api/settings/api-keys (the same endpoint the API Keys panel
    already uses), so we don't add a new route just for this read.
  * Added: migrateLegacySentinelBrowserKeys() — one-shot, idempotent
    helper that clears sb_sentinel_client_id / _secret / _instance_id
    from BOTH localStorage and sessionStorage. We deliberately do NOT
    auto-POST those legacy browser values to the backend; doing so
    would silently migrate a secret across a trust boundary without
    operator consent. Operators re-enter once in the API Keys panel
    and the legacy keys get wiped here.
  * fetchSentinelTile and getSentinelToken no longer send client_id /
    client_secret in the request body. The backend uses .env.

frontend/src/components/SettingsPanel.tsx
  * Dropped sb_sentinel_client_id / _secret / _instance_id from
    PRIVACY_SENSITIVE_BROWSER_KEYS — they're no longer written.
  * SentinelTab rewritten: removed the inline Client ID / Client Secret
    inputs + Save / Clear / Test buttons. Replaced with a status panel
    that calls checkBackendSentinelStatus() on mount, a one-click
    "Open API Keys Panel" button, and a migration banner that appears
    only when migrateLegacySentinelBrowserKeys() actually cleared
    something.
  * Setup guide STEP 3 now points to the API Keys panel instead of
    the local form.

frontend/src/app/page.tsx
  * Added a one-time useEffect that fires checkBackendSentinelStatus()
    on mount so the cached value (which the synchronous
    hasSentinelCredentials() shim reads) is populated before
    MaplibreViewer's tile-URL memo runs.

Tests
-----
backend/tests/test_sentinel_credentials_server_side.py (new)
  * API_REGISTRY surface — sentinel_client_id / sentinel_client_secret
    are registered with the right env_keys, ALLOWED_ENV_KEYS lets
    /api/settings/api-keys PUT them.
  * Resolution order — body wins, env is fallback, neither → 400 with
    the friendly pointer message, and NO upstream HTTP call when
    neither source has credentials (asserted via
    MagicMock(side_effect=AssertionError)).
  * /api/sentinel/tile same shape.

frontend/src/__tests__/utils/sentinelHub.test.ts (new)
  * migrateLegacySentinelBrowserKeys clears localStorage AND
    sessionStorage, reports what it cleared, idempotent.
  * fetchSentinelTile + getSentinelToken POST WITHOUT client_id /
    client_secret in the body (plants leaked credentials in browser
    storage first to prove they are NOT picked up).
  * checkBackendSentinelStatus parses /api/settings/api-keys correctly:
    true only when both keys is_set, false on partial config or
    network errors.

All 7 backend tests + 8 frontend tests pass locally. The
test_no_new_duplicate_routes guard and the api-settings test suite
still pass.

Credit: @tg12 for the audit report.

frontend/src/__tests__/utils/sentinelHub.test.ts

@@ -0,0 +1,169 @@
+/**
+ * Issue #298 (tg12): Sentinel credentials must no longer live in browser
+ * storage, and the proxy calls must not forward them in request bodies.
+ * These tests pin both invariants on ``lib/sentinelHub``:
+ *
+ *  1. ``migrateLegacySentinelBrowserKeys()`` clears the legacy keys
+ *     idempotently and reports what it cleared.
+ *  2. ``fetchSentinelTile()`` and ``getSentinelToken()`` POST WITHOUT
+ *     ``client_id`` or ``client_secret`` in the body — the backend
+ *     resolves credentials from its ``.env``. A future refactor that
+ *     accidentally re-introduces browser-storage reads (e.g. by
+ *     restoring ``getSentinelCredentials()`` and forwarding it) gets a
+ *     loud test failure here rather than a silent privacy regression.
+ *  3. ``checkBackendSentinelStatus()`` queries ``/api/settings/api-keys``
+ *     and returns true only when both Sentinel keys report ``is_set``.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import {
+  migrateLegacySentinelBrowserKeys,
+  fetchSentinelTile,
+  getSentinelToken,
+  checkBackendSentinelStatus,
+  refreshSentinelStatus,
+} from '@/lib/sentinelHub';
+
+const originalFetch = globalThis.fetch;
+
+describe('lib/sentinelHub — issue #298 server-side credentials', () => {
+  beforeEach(() => {
+    window.localStorage.clear();
+    window.sessionStorage.clear();
+    refreshSentinelStatus();
+  });
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+    window.localStorage.clear();
+    window.sessionStorage.clear();
+    refreshSentinelStatus();
+  });
+
+  describe('migrateLegacySentinelBrowserKeys', () => {
+    it('clears legacy localStorage keys and reports what it cleared', () => {
+      window.localStorage.setItem('sb_sentinel_client_id', 'sh-leaked-id');
+      window.localStorage.setItem('sb_sentinel_client_secret', 'leaked-secret');
+      window.localStorage.setItem('sb_sentinel_instance_id', 'leaked-instance');
+
+      const result = migrateLegacySentinelBrowserKeys();
+
+      expect(window.localStorage.getItem('sb_sentinel_client_id')).toBeNull();
+      expect(window.localStorage.getItem('sb_sentinel_client_secret')).toBeNull();
+      expect(window.localStorage.getItem('sb_sentinel_instance_id')).toBeNull();
+      expect(result.cleared.sort()).toEqual([
+        'sb_sentinel_client_id',
+        'sb_sentinel_client_secret',
+        'sb_sentinel_instance_id',
+      ].sort());
+    });
+
+    it('clears sessionStorage too (privacy-strict mode used to put them there)', () => {
+      window.sessionStorage.setItem('sb_sentinel_client_id', 'sh-session-id');
+      window.sessionStorage.setItem('sb_sentinel_client_secret', 'session-secret');
+
+      const result = migrateLegacySentinelBrowserKeys();
+
+      expect(window.sessionStorage.getItem('sb_sentinel_client_id')).toBeNull();
+      expect(window.sessionStorage.getItem('sb_sentinel_client_secret')).toBeNull();
+      expect(result.cleared).toContain('sb_sentinel_client_id');
+      expect(result.cleared).toContain('sb_sentinel_client_secret');
+    });
+
+    it('is idempotent — calling it on a clean store reports nothing cleared', () => {
+      const result = migrateLegacySentinelBrowserKeys();
+      expect(result.cleared).toEqual([]);
+    });
+  });
+
+  describe('proxy requests no longer forward credentials', () => {
+    it('fetchSentinelTile POSTs without client_id/client_secret in the body', async () => {
+      // Plant credentials in browser storage to prove they would NOT be
+      // picked up even if present. Pre-#298, this would have been read
+      // from localStorage and posted in the body.
+      window.localStorage.setItem('sb_sentinel_client_id', 'sh-leaked-id');
+      window.localStorage.setItem('sb_sentinel_client_secret', 'leaked-secret');
+
+      const fetchMock = vi.fn(async () => new Response(new ArrayBuffer(0), { status: 200 }));
+      globalThis.fetch = fetchMock as unknown as typeof globalThis.fetch;
+
+      await fetchSentinelTile(6, 30, 20, 'TRUE-COLOR', '2026-01-01');
+
+      expect(fetchMock).toHaveBeenCalledTimes(1);
+      const [, init] = fetchMock.mock.calls[0] as [unknown, RequestInit];
+      const body = JSON.parse(String(init.body));
+      expect(body).not.toHaveProperty('client_id');
+      expect(body).not.toHaveProperty('client_secret');
+      // Sanity: the legitimate fields are still there.
+      expect(body).toMatchObject({ preset: 'TRUE-COLOR', date: '2026-01-01', z: 6, x: 30, y: 20 });
+    });
+
+    it('getSentinelToken POSTs with an empty form body (backend uses env)', async () => {
+      window.localStorage.setItem('sb_sentinel_client_id', 'sh-leaked-id');
+      window.localStorage.setItem('sb_sentinel_client_secret', 'leaked-secret');
+
+      const fetchMock = vi.fn(async () =>
+        new Response(JSON.stringify({ access_token: 'stub', expires_in: 300 }), { status: 200 }),
+      );
+      globalThis.fetch = fetchMock as unknown as typeof globalThis.fetch;
+
+      const token = await getSentinelToken();
+
+      expect(token).toBe('stub');
+      expect(fetchMock).toHaveBeenCalledTimes(1);
+      const [, init] = fetchMock.mock.calls[0] as [unknown, RequestInit];
+      const body = String(init.body);
+      // Body is a URLSearchParams stringification. We assert that the
+      // leaked credential never appears in it.
+      expect(body).not.toContain('sh-leaked-id');
+      expect(body).not.toContain('leaked-secret');
+    });
+  });
+
+  describe('checkBackendSentinelStatus', () => {
+    it('returns true when both Sentinel keys report is_set on /api/settings/api-keys', async () => {
+      const fetchMock = vi.fn(async (input: unknown) => {
+        const url = String(input);
+        if (url.endsWith('/api/settings/api-keys')) {
+          return new Response(
+            JSON.stringify([
+              { id: 'sentinel_client_id', env_key: 'SENTINEL_CLIENT_ID', is_set: true },
+              { id: 'sentinel_client_secret', env_key: 'SENTINEL_CLIENT_SECRET', is_set: true },
+              { id: 'opensky_client_id', env_key: 'OPENSKY_CLIENT_ID', is_set: false },
+            ]),
+            { status: 200 },
+          );
+        }
+        return new Response('not found', { status: 404 });
+      });
+      globalThis.fetch = fetchMock as unknown as typeof globalThis.fetch;
+
+      const configured = await checkBackendSentinelStatus();
+      expect(configured).toBe(true);
+    });
+
+    it('returns false when only one of the two keys is set', async () => {
+      const fetchMock = vi.fn(async () =>
+        new Response(
+          JSON.stringify([
+            { id: 'sentinel_client_id', env_key: 'SENTINEL_CLIENT_ID', is_set: true },
+            { id: 'sentinel_client_secret', env_key: 'SENTINEL_CLIENT_SECRET', is_set: false },
+          ]),
+          { status: 200 },
+        ),
+      );
+      globalThis.fetch = fetchMock as unknown as typeof globalThis.fetch;
+
+      const configured = await checkBackendSentinelStatus();
+      expect(configured).toBe(false);
+    });
+
+    it('fails safely (false) when the backend errors', async () => {
+      const fetchMock = vi.fn(async () => { throw new Error('network down'); });
+      globalThis.fetch = fetchMock as unknown as typeof globalThis.fetch;
+
+      const configured = await checkBackendSentinelStatus();
+      expect(configured).toBe(false);
+    });
+  });
+});