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/services/classifier.py

@@ -9,6 +9,10 @@ wrapper and from API route handlers that already hold a session.
 Updated in Plan 03-03: classify_document uses load_topics_for_user (D-17) to scope
 topic lookup to the document owner's namespace, and creates AI-suggested topics in
 the user's namespace via create_topic(user_id=doc.user_id) (D-11).
+
+Updated in Plan 03-04: classify_document and suggest_topics_for_document now accept
+ai_provider and ai_model kwargs. No longer calls storage.load_settings(). Provider
+resolved via get_provider() using per-user settings from DB (D-14, D-15).
 """
 from __future__ import annotations
 
@@ -16,30 +20,48 @@ import uuid as _uuid
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from config import settings as app_settings
 from db.models import Document
 from services import storage
 from ai import get_provider
 
 MAX_AI_CHARS = 8_000
 
+_DEFAULT_SYSTEM_PROMPT = """You are a document classification assistant. When given a document's text content and a list of existing topics, you must:
+1. Assign the document to one or more relevant topics from the list.
+2. If no existing topics fit well, suggest new topic names.
+Return ONLY valid JSON in this exact format, with no additional text or explanation:
+{"assigned_topics": ["topic1"], "new_topic_suggestions": ["new topic name"]}
+If the document fits no topics and you have no suggestions, return: {"assigned_topics": [], "new_topic_suggestions": []}"""
+
 
 async def classify_document(
     session: AsyncSession,
     doc_id: str,
     topic_names: list[str] | None = None,
+    ai_provider: str | None = None,
+    ai_model: str | None = None,
 ) -> list[str]:
     """
     Classify a document by its ID. Returns the list of assigned topic names.
     If topic_names is provided, restrict classification to those topics.
     Auto-creates any newly suggested topics in the document owner's namespace (D-11).
+
+    ai_provider and ai_model come from the document owner's User record (D-14).
+    Falls back to app_settings.default_ai_provider / default_ai_model when None (D-15).
     """
     meta = await storage.get_metadata(session, doc_id)
     if meta is None:
         raise ValueError(f"Document {doc_id} not found")
 
-    settings = storage.load_settings()
-    system_prompt = settings.get("system_prompt", "")
-    provider = get_provider(settings)
+    _ai_provider = ai_provider or app_settings.default_ai_provider
+    _ai_model = ai_model or app_settings.default_ai_model
+    system_prompt = app_settings.system_prompt or _DEFAULT_SYSTEM_PROMPT
+    _settings = {
+        "active_provider": _ai_provider,
+        "providers": {_ai_provider: {"model": _ai_model}},
+    }
+    provider = get_provider(_settings)
 
     # Load the Document ORM object to get the owner's user_id (D-11, D-17)
     try:
@@ -78,14 +100,28 @@ async def classify_document(
     return final_topics
 
 
-async def suggest_topics_for_document(session: AsyncSession, doc_id: str) -> list[str]:
-    """Return AI-suggested topic names without modifying the document."""
+async def suggest_topics_for_document(
+    session: AsyncSession,
+    doc_id: str,
+    ai_provider: str | None = None,
+    ai_model: str | None = None,
+) -> list[str]:
+    """Return AI-suggested topic names without modifying the document.
+
+    ai_provider and ai_model come from the document owner's User record (D-14).
+    Falls back to app_settings.default_ai_provider / default_ai_model when None (D-15).
+    """
     meta = await storage.get_metadata(session, doc_id)
     if meta is None:
         raise ValueError(f"Document {doc_id} not found")
 
-    settings = storage.load_settings()
-    system_prompt = settings.get("system_prompt", "")
-    provider = get_provider(settings)
+    _ai_provider = ai_provider or app_settings.default_ai_provider
+    _ai_model = ai_model or app_settings.default_ai_model
+    system_prompt = app_settings.system_prompt or _DEFAULT_SYSTEM_PROMPT
+    _settings = {
+        "active_provider": _ai_provider,
+        "providers": {_ai_provider: {"model": _ai_model}},
+    }
+    provider = get_provider(_settings)
     text = meta.get("extracted_text", "")
     return await provider.suggest_topics(text[:MAX_AI_CHARS], system_prompt)