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/replay/api.py

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

backend/app/replay/transformer.py

@@ -2,11 +2,11 @@
 
 from __future__ import annotations
 
-import logging
+import structlog
 
 from app.replay.models import ReplayStep, StepType
 
-logger = logging.getLogger(__name__)
+logger = structlog.get_logger()
 
 _EMPTY_TIMESTAMP = "1970-01-01T00:00:00Z"