Follow up on #375 review: dedupe live-data route and harden serializers.

Align full /api/live-data with slow-tier orjson options, remove dead main.py duplicate, cap slow batches to pool size, cancel queued work on timeout, and stop retrying HTTP 4xx/5xx.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/data/duplicate_routes_baseline.json

@@ -38,10 +38,6 @@
       "main",
       "routers.health"
     ],
-    "GET /api/live-data": [
-      "main",
-      "routers.data"
-    ],
     "GET /api/live-data/fast": [
       "main",
       "routers.data"

backend/tests/test_issue_375_production_hardening.py

@@ -65,3 +65,68 @@ class TestLiveDataFullEndpoint:
         r2 = client.get("/api/live-data", headers={"If-None-Match": etag})
         assert r2.status_code == 304
         assert r2.headers.get("etag") == etag
+
+    def test_live_data_serializes_non_json_native_values(self, client):
+        from datetime import datetime, timezone
+
+        from services.fetchers import _store
+
+        with _store._data_lock:
+            prior = _store.latest_data.get("gdelt")
+            _store.latest_data["gdelt"] = [
+                {"observed": datetime(2026, 1, 1, tzinfo=timezone.utc)},
+            ]
+        try:
+            r = client.get("/api/live-data")
+            assert r.status_code == 200
+            assert "2026-01-01" in r.text
+        finally:
+            with _store._data_lock:
+                _store.latest_data["gdelt"] = prior
+
+
+class TestSlowTaskConcurrency:
+    def test_run_tasks_caps_batch_size_to_executor_workers(self, monkeypatch):
+        from unittest.mock import MagicMock
+
+        import services.data_fetcher as df
+
+        class _FakeExecutor:
+            _max_workers = 2
+
+            def submit(self, func):
+                return MagicMock()
+
+        mock_executor = _FakeExecutor()
+        monkeypatch.setattr(df, "_executor_for_task_label", lambda _label: mock_executor)
+        monkeypatch.setattr(df, "_SLOW_FETCH_CONCURRENCY", 8)
+
+        batch_sizes = []
+
+        def _capture_drain(_label, futures):
+            batch_sizes.append(len(futures))
+
+        monkeypatch.setattr(df, "_drain_task_futures", _capture_drain)
+
+        jobs = [lambda: None for _ in range(5)]
+        df._run_tasks("slow-tier-test", jobs)
+
+        assert batch_sizes == [2, 2, 1]
+
+
+class TestFetcherRetryScope:
+    def test_http_error_is_not_retried(self, monkeypatch):
+        import requests
+
+        from services.fetchers.retry import with_retry
+
+        attempts = {"n": 0}
+
+        @with_retry(max_retries=2, base_delay=0.01)
+        def _raises_http():
+            attempts["n"] += 1
+            raise requests.HTTPError("403 Client Error")
+
+        with pytest.raises(requests.HTTPError):
+            _raises_http()
+        assert attempts["n"] == 1