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/storage.py

@@ -9,11 +9,8 @@ Public function names are PRESERVED from the old flat-file implementation so
 that api/documents.py and api/topics.py can be updated in Plan 05 with minimal
 changes (async def + await + session parameter).
 
-Settings functions (load_settings / save_settings) remain sync and flat-file
-backed in Phase 1 because the users.ai_provider / users.ai_model schema columns
-cannot be populated until Phase 2.
-# Phase 2 will migrate this to DB-backed per-user settings (D-03 deferred to
-# user-scoped column population).
+Phase 3 D-12: load_settings / save_settings / mask_api_key / settings_masked removed.
+All AI config comes from DB (users.ai_provider / users.ai_model set by admin).
 
 D-05: Storage service layer switched to PostgreSQL + MinIO.
 D-06: Object key schema: {user_id}/{document_id}/{uuid4()}{ext} — human filename in DB only.
@@ -21,8 +18,6 @@ D-03: documents.user_id is None (nullable) in Phase 1 — no auth system yet.
 """
 from __future__ import annotations
 
-import copy
-import json
 import sys
 import uuid
 from datetime import datetime, timezone
@@ -32,7 +27,6 @@ from sqlalchemy import select, delete, text, or_
 from sqlalchemy import func as sql_func
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from config import DEFAULT_SETTINGS, SETTINGS_FILE
 from db.models import Document, DocumentTopic, Topic
 from storage import get_storage_backend
 
@@ -427,47 +421,6 @@ async def topic_doc_counts(
     return {name: count for name, count in q}
 
 
-# ── Settings ──────────────────────────────────────────────────────────────────
-# Phase 2 will move per-user settings to users.ai_provider / users.ai_model
-# (D-03 deferred to user-scoped column population).
-# For now these remain as flat-file JSON — single-writer, no filelock needed.
-
-def load_settings() -> dict:
-    """Read app settings from the flat-file SETTINGS_FILE.
-
-    Falls back to DEFAULT_SETTINGS if the file is missing.
-    # Phase 2 will move per-user settings to users.ai_provider / users.ai_model.
-    """
-    try:
-        return json.loads(SETTINGS_FILE.read_text())
-    except (FileNotFoundError, json.JSONDecodeError):
-        return copy.deepcopy(DEFAULT_SETTINGS)
-
-
-def save_settings(settings: dict) -> None:
-    """Write app settings to the flat-file SETTINGS_FILE.
-
-    No filelock — Phase 1 settings file is single-writer.
-    # Phase 2 will move per-user settings to users.ai_provider / users.ai_model.
-    """
-    SETTINGS_FILE.write_text(json.dumps(settings, indent=2))
-
-
-def mask_api_key(key: str) -> str:
-    if not key or len(key) <= 4:
-        return "****"
-    return "****" + key[-4:]
-
-
-def settings_masked(settings: dict) -> dict:
-    s = copy.deepcopy(settings)
-    for prov in ("anthropic", "openai"):
-        key = s.get("providers", {}).get(prov, {}).get("api_key", "")
-        if key:
-            s["providers"][prov]["api_key"] = mask_api_key(key)
-    return s
-
-
 # ── Public surface ─────────────────────────────────────────────────────────────
 
 __all__ = [
@@ -485,8 +438,4 @@ __all__ = [
     "update_topic",
     "delete_topic",
     "topic_doc_counts",
-    "load_settings",
-    "save_settings",
-    "mask_api_key",
-    "settings_masked",
 ]