test: close coverage gaps and add frontend test infrastructure

Backend (516 tests, 94% coverage):
- Add azure_openai endpoint/deployment validation tests (config.py -> 100%)
- Add _total_conversations and _avg_turns direct tests (queries.py -> 100%)
- Add transformer edge cases: list content, string checkpoint, invalid JSON,
  malformed message graceful skip (transformer.py -> 93%)
- Add safety combined status_code+error_message interaction tests
- Fix ambiguous 200/422 assertion to strict 422
- Add E2E pagination shape assertions (total, page, per_page, row count)
- Fix ReplayPool mock to respect LIMIT/OFFSET params

Frontend (23 tests, vitest + happy-dom + @testing-library/react):
- Add vitest infrastructure with happy-dom environment
- Add api.ts tests: success, HTTP error, success=false, URL encoding
- Add DashboardPage tests: loading, data, error, empty states
- Add ReplayListPage tests: loading, empty, data, error, status badge classes
- Add ReplayPage tests: loading, steps, empty, error states

backend/tests/e2e/test_replay_analytics.py

@@ -47,7 +47,13 @@ class ReplayPool(FakePool):
             if "COUNT" in query and "conversations" in query:
                 return FakeCursor([(len(self._convos),)])
             if "conversations" in query and "SELECT" in query:
-                return FakeCursor(self._convos)
+                # Respect LIMIT/OFFSET from params if provided
+                rows = self._convos
+                if params:
+                    offset = params.get("offset", 0)
+                    limit = params.get("limit", len(rows))
+                    rows = rows[offset : offset + limit]
+                return FakeCursor(rows)
             if "checkpoints" in query:
                 return FakeCursor(self._checkpoints)
             # Analytics queries
@@ -122,6 +128,11 @@ class TestFlow6ReplayConversation:
             assert resp.status_code == 200
             body = resp.json()
             assert body["success"] is True
+            data = body["data"]
+            assert data["total"] == 5
+            assert data["page"] == 1
+            assert data["per_page"] == 2
+            assert len(data["conversations"]) == 2
 
     def test_replay_thread_not_found(self) -> None:
         pool = ReplayPool(checkpoints=[])