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/unit/test_main.py

@@ -36,7 +36,7 @@ class TestMainModule:
 
     def test_health_route_registered(self) -> None:
         routes = [r.path for r in app.routes if hasattr(r, "path")]
-        assert "/api/health" in routes
+        assert "/api/v1/health" in routes
 
     def test_app_version_is_0_5_0(self) -> None:
         assert app.version == "0.6.0"