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/app/analytics/api.py

@@ -16,7 +16,7 @@ if TYPE_CHECKING:
     from psycopg_pool import AsyncConnectionPool
 
 router = APIRouter(
-    prefix="/api/analytics",
+    prefix="/api/v1/analytics",
     tags=["analytics"],
     dependencies=[Depends(require_admin_api_key)],
 )