feat: migrate app_config volume to storage-service config bucket (Phase 3)

All JSON config files (AI settings, doc settings, appearance, themes) now live
in the 'config' bucket of storage-service instead of a shared Docker volume.

- backend/core/config_storage.py: new async HTTP helpers for config bucket r/w
- backend/core/app_config.py: fully async rewrite; all load_*/save_*/seed_*
  functions use config_storage instead of filesystem
- backend/routers/settings.py: all asyncio.to_thread() wrappers removed; direct
  await calls throughout; update_theme reads via load_theme_by_id()
- backend/main.py: await seed_builtin_themes() directly (no to_thread)
- ai-service: remove CONFIG_PATH, add STORAGE_SERVICE_URL; config_reader now
  fetches from storage-service via httpx
- doc-service: config_reader rewritten to fetch/write via storage-service
- docker-compose: remove app_config volume; add storage-service depends_on for
  ai-service; remove DATA_DIR and CONFIG_PATH from doc-service

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -15,7 +15,7 @@ from app.services.service_health import check_all, health_check_loop, register_s
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    await asyncio.to_thread(seed_builtin_themes)
+    await seed_builtin_themes()
     register_services(
         doc_service_url=settings.DOC_SERVICE_URL,
         ai_service_url=settings.AI_SERVICE_URL,