Fix #218/#219/#220: identify ShadowBroker on Wikipedia + Wikidata calls (#284)

Wikimedia's User-Agent policy asks API clients to identify themselves
with a stable, contactable identifier so their operators can rate-limit
or coordinate. Before this change, ShadowBroker was sending:

- Backend (region_dossier.py): generic project default UA only; no
  Api-User-Agent.
- Frontend (useRegionDossier.ts, WikiImage.tsx, NewsFeed.tsx): zero
  identifying header at all; three separate copy-pasted anonymous
  fetches with their own module-local caches.

Three separate components doing the same broken thing meant policy
fixes had to happen in three places, with no shared cache or kill
switch.

Fix (no UX change, zero hostility):

== Backend ==

`backend/services/region_dossier.py` now sets explicit `User-Agent` +
`Api-User-Agent` headers on every outbound Wikidata and Wikipedia
request via a new `_WIKIMEDIA_REQUEST_HEADERS` constant. The identifier
includes a contact path (issues page on the public GitHub repo).

== Frontend ==

New shared helper `frontend/src/lib/wikimediaClient.ts`:
- `fetchWikipediaSummary(title)` — single source of truth for Wikipedia
  REST summary lookups, with one shared LRU cache (in-flight requests
  deduplicated, 512-entry cap), `Api-User-Agent` on every fetch.
- `fetchWikidataSparql(query)` — same shape for Wikidata SPARQL.
- `WIKIMEDIA_API_USER_AGENT` — exported constant; one place to update
  if Wikimedia ever asks us to back off.

Refactored three components to use the shared client:
- `frontend/src/hooks/useRegionDossier.ts` — fetchLeader() and
  fetchLocalWikiSummary() now route through the shared helpers.
- `frontend/src/components/WikiImage.tsx` — uses fetchWikipediaSummary,
  proper React state instead of module-mutation + forceUpdate trick.
- `frontend/src/components/NewsFeed.tsx` — same shape.

UX: byte-for-byte identical. Same thumbnails, same dossier content,
same load behavior. The only observable difference is the outgoing
request header.

Note on #239 (route duplication): an audit-grade inventory shows 166
main.py routes are shadowed by router modules. That cleanup is too
large to land safely in this PR; it will be staged as a separate
ladder of small PRs grouped by router module.

Tests:
- `backend/tests/test_region_dossier_wikimedia_ua.py` — 3 tests
  asserting backend headers are present.
- `frontend/src/__tests__/utils/wikimediaClient.test.ts` — 9 tests
  covering Api-User-Agent presence, shared cache, concurrent
  deduplication, disambiguation/HTTP-error/network-error fallthroughs,
  empty-input safety.

Local: backend 76/76 security suite green, frontend 716/716 vitest
suite green.

Credit: tg12 (external security audit).

frontend/src/hooks/useRegionDossier.ts

@@ -1,5 +1,6 @@
 import { useCallback, useState, useEffect } from 'react';
 import type { RegionDossier, SelectedEntity } from '@/types/dashboard';
+import { fetchWikipediaSummary, fetchWikidataSparql } from '@/lib/wikimediaClient';
 
 // ─── CACHE ─────────────────────────────────────────────────────────────────
 // Simple in-memory cache keyed by rounded lat/lng (0.1° ≈ 11km grid), 24h TTL.
@@ -114,7 +115,11 @@ async function fetchCountryData(countryCode: string) {
   return Array.isArray(data) ? data[0] || {} : data || {};
 }
 
-/** Fetch head of state + government type from Wikidata SPARQL (direct browser call). */
+/** Fetch head of state + government type from Wikidata SPARQL.
+ *
+ * Issue #218 (tg12): routes through lib/wikimediaClient so the
+ * Api-User-Agent header is set per Wikimedia's UA policy.
+ */
 async function fetchLeader(countryName: string) {
   if (!countryName) return { leader: 'Unknown', government_type: 'Unknown' };
   const safeName = countryName.replace(/"/g, '\\"').replace(/'/g, "\\'");
@@ -127,13 +132,11 @@ async function fetchLeader(countryName: string) {
       SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
     } LIMIT 1
   `;
-  const url = `https://query.wikidata.org/sparql?query=${encodeURIComponent(sparql)}&format=json`;
-  const res = await fetch(url, {
-    headers: { Accept: 'application/sparql-results+json' },
-  });
-  if (!res.ok) throw new Error(`Wikidata HTTP ${res.status}`);
-  const results = (await res.json()).results?.bindings || [];
-  if (results.length > 0) {
+  const results = await fetchWikidataSparql<{
+    leaderLabel?: { value: string };
+    govTypeLabel?: { value: string };
+  }>(sparql);
+  if (results && results.length > 0) {
     return {
       leader: results[0].leaderLabel?.value || 'Unknown',
       government_type: results[0].govTypeLabel?.value || 'Unknown',
@@ -142,27 +145,25 @@ async function fetchLeader(countryName: string) {
   return { leader: 'Unknown', government_type: 'Unknown' };
 }
 
-/** Fetch Wikipedia summary for a place (direct browser call). */
+/** Fetch Wikipedia summary for a place.
+ *
+ * Issue #219 (tg12): routes through lib/wikimediaClient so the
+ * Api-User-Agent header is set per Wikimedia's UA policy, AND the
+ * shared cache means consecutive useRegionDossier + WikiImage +
+ * NewsFeed lookups for the same article all hit the same slot.
+ */
 async function fetchLocalWikiSummary(placeName: string, countryName = '') {
   if (!placeName) return {};
   const candidates = [placeName];
   if (countryName) candidates.push(`${placeName}, ${countryName}`);
-
   for (const name of candidates) {
-    try {
-      const slug = encodeURIComponent(name.replace(/ /g, '_'));
-      const url = `https://en.wikipedia.org/api/rest_v1/page/summary/${slug}`;
-      const res = await fetch(url);
-      if (!res.ok) continue;
-      const data = await res.json();
-      if (data.type === 'disambiguation') continue;
+    const summary = await fetchWikipediaSummary(name);
+    if (summary) {
       return {
-        description: data.description || '',
-        extract: data.extract || '',
-        thumbnail: data.thumbnail?.source || '',
+        description: summary.description,
+        extract: summary.extract,
+        thumbnail: summary.thumbnail,
       };
-    } catch {
-      continue;
     }
   }
   return {};