feat(02-02): auth API endpoints + security hardening + Python 3.9 compat

- backend/api/auth.py: register, login (TOTP+backup), refresh, logout,
  me, change-password; per-account Redis rate limit; HIBP check
- backend/main.py: Origin validation middleware, CSP headers middleware,
  CORS locked to settings.cors_origins, Redis lifespan (app.state.redis),
  admin bootstrap, auth router included, slowapi SlowAPIMiddleware
- backend/services/email.py: already created in Plan 01 (verified exists)
- Python 3.9 compat: fixed match statement in ai/__init__.py,
  str|None union syntax in openai_provider.py, api/documents.py,
  api/topics.py, api/settings.py, services/classifier.py

All 17 tests in test_auth_api.py pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/documents.py

@@ -1,4 +1,5 @@
 from datetime import datetime, timezone
+from typing import Optional
 
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, UploadFile
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -68,7 +69,7 @@ async def upload_document(
 
 @router.get("")
 async def list_documents(
-    topic: str | None = Query(None),
+    topic: Optional[str] = Query(None),
     page: int = Query(1, ge=1),
     per_page: int = Query(20, ge=1, le=100),
     session: AsyncSession = Depends(get_db),