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/api/settings.py

@@ -1,86 +0,0 @@
-import time
-from typing import Optional
-
-from fastapi import APIRouter, HTTPException
-from pydantic import BaseModel
-from services import storage
-from config import DEFAULT_SYSTEM_PROMPT
-from ai import get_provider
-
-router = APIRouter(prefix="/api/settings", tags=["settings"])
-
-
-class SettingsPatch(BaseModel):
-    system_prompt: Optional[str] = None
-    active_provider: Optional[str] = None
-    providers: Optional[dict] = None
-
-
-class TestProviderRequest(BaseModel):
-    provider: str
-
-
-@router.get("")
-async def get_settings():
-    settings = storage.load_settings()
-    return storage.settings_masked(settings)
-
-
-@router.patch("")
-async def patch_settings(body: SettingsPatch):
-    settings = storage.load_settings()
-
-    if body.system_prompt is not None:
-        settings["system_prompt"] = body.system_prompt
-
-    if body.active_provider is not None:
-        valid = {"anthropic", "openai", "ollama", "lmstudio"}
-        if body.active_provider not in valid:
-            raise HTTPException(400, f"Invalid provider. Must be one of: {valid}")
-        settings["active_provider"] = body.active_provider
-
-    if body.providers is not None:
-        # Deep merge per-provider config
-        for prov_name, prov_cfg in body.providers.items():
-            if prov_name not in settings.get("providers", {}):
-                settings.setdefault("providers", {})[prov_name] = {}
-            existing = settings["providers"][prov_name]
-            for key, val in prov_cfg.items():
-                # Don't overwrite api_key if it comes in masked (contains ****)
-                if key == "api_key" and val and "****" in str(val):
-                    continue
-                existing[key] = val
-
-    storage.save_settings(settings)
-    return storage.settings_masked(settings)
-
-
-@router.post("/test-provider")
-async def test_provider(body: TestProviderRequest):
-    settings = storage.load_settings()
-    # Temporarily switch active provider for the test
-    test_settings = dict(settings)
-    test_settings["active_provider"] = body.provider
-
-    try:
-        provider = get_provider(test_settings)
-    except ValueError as e:
-        raise HTTPException(400, str(e))
-
-    start = time.monotonic()
-    try:
-        ok = await provider.health_check()
-    except Exception as e:
-        return {"ok": False, "message": str(e), "latency_ms": 0}
-
-    latency_ms = int((time.monotonic() - start) * 1000)
-    return {
-        "ok": ok,
-        "message": "Connection successful" if ok else "Health check failed",
-        "latency_ms": latency_ms,
-    }
-
-
-@router.get("/default-prompt")
-async def get_default_prompt():
-    return {"system_prompt": DEFAULT_SYSTEM_PROMPT}