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/pyproject.toml

@@ -21,6 +21,8 @@ dependencies = [
     "python-dotenv>=1.0,<2.0",
     "httpx>=0.28,<1.0",
     "openapi-spec-validator>=0.7,<1.0",
+    "alembic>=1.13,<2.0",
+    "structlog>=24.0,<26.0",
 ]
 
 [project.optional-dependencies]