refactor: engineering improvements -- API versioning, structured logging, Alembic, error standardization, test coverage

- API versioning: all REST endpoints prefixed with /api/v1/
- Structured logging: replaced stdlib logging with structlog (console/JSON modes)
- Alembic migrations: versioned DB schema with initial migration
- Error standardization: global exception handlers for consistent envelope format
- Interrupt cleanup: asyncio background task for expired interrupt removal
- Integration tests: +30 tests (analytics, replay, openapi, error, session APIs)
- Frontend tests: +57 tests (all components, pages, useWebSocket hook)
- Backend: 557 tests, 89.75% coverage | Frontend: 80 tests, 16 test files

backend/tests/e2e/test_chat_flows.py

@@ -341,7 +341,7 @@ class TestChatEdgeCases:
     def test_health_endpoint(self) -> None:
         app = create_e2e_app()
         with TestClient(app) as client:
-            resp = client.get("/api/health")
+            resp = client.get("/api/v1/health")
             assert resp.status_code == 200
             assert resp.json()["status"] == "ok"