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/conftest.py

@@ -174,7 +174,7 @@ def create_e2e_app(
     app.state.analytics_recorder = AsyncMock()
     app.state.conversation_tracker = AsyncMock()
 
-    @app.get("/api/health")
+    @app.get("/api/v1/health")
     def health_check() -> dict:
         return {"status": "ok", "version": "test"}