feat(03-04): retire flat-file settings; wire per-user AI config via DB lookup

- config.py: Remove SETTINGS_FILE, DEFAULT_SYSTEM_PROMPT, DEFAULT_SETTINGS
  constants; add system_prompt, default_ai_provider, default_ai_model to Settings
- services/classifier.py: Add _DEFAULT_SYSTEM_PROMPT module constant; classify_document
  and suggest_topics_for_document accept ai_provider/ai_model kwargs; no longer calls
  storage.load_settings() — uses app_settings defaults with DB-supplied overrides (D-14, D-15)
- services/storage.py: Delete load_settings, save_settings, mask_api_key, settings_masked;
  remove from __all__; remove import copy, json, DEFAULT_SETTINGS, SETTINGS_FILE (D-12)
- tasks/document_tasks.py: _run resolves user.ai_provider/ai_model via session.get(User,
  doc.user_id) and passes through to classifier; task signature unchanged (T-03-19)
- api/settings.py: Deleted — /api/settings endpoint removed (D-12)
- main.py: Remove settings_router import and include_router call
- tests/test_settings.py: Replace all tests with test_settings_endpoint_removed (404, green)
- tests/test_classifier.py: Implement test_per_user_provider, test_celery_task_uses_user_provider,
  test_default_provider_fallback; remove xfail markers (DOC-03, DOC-05)

backend/main.py

@@ -15,7 +15,6 @@ from starlette.responses import Response as StarletteResponse
 
 from api.auth import limiter as auth_limiter
 from api.documents import router as documents_router
-from api.settings import router as settings_router
 from api.topics import router as topics_router
 from config import settings
 from db.session import AsyncSessionLocal, engine
@@ -171,7 +170,6 @@ async def health(request: Request):
 
 app.include_router(documents_router)
 app.include_router(topics_router)
-app.include_router(settings_router)
 
 # Phase 2: auth and admin routers
 from api.auth import router as auth_router  # noqa: E402