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/auth.py

@@ -2,14 +2,14 @@
 
 from __future__ import annotations
 
-import logging
 import secrets
 from typing import Annotated
 
+import structlog
 from fastapi import Depends, HTTPException, Query, Request, WebSocket, status
 from fastapi.security import APIKeyHeader
 
-logger = logging.getLogger(__name__)
+logger = structlog.get_logger()
 
 _API_KEY_HEADER = APIKeyHeader(name="X-API-Key", auto_error=False)