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/ai/openai_provider.py

@@ -7,7 +7,7 @@ MAX_AI_CHARS = 8_000
 
 
 class OpenAIProvider(AIProvider):
-    def __init__(self, api_key: str, model: str = "gpt-4o", base_url: str | None = None):
+    def __init__(self, api_key: str, model: str = "gpt-4o", base_url=None):  # type: ignore[type-arg]
         self._api_key = api_key
         self._model = model
         self._base_url = base_url