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>

features/doc-service/app/services/config_reader.py

@@ -1,19 +1,20 @@
 """
-Reads doc_service_config.json from the shared config volume.
+Reads doc_service_config.json from the storage-service config bucket.
 30-second TTL cache + env var overrides.
 
 Env var overrides (all optional):
   DOC_MAX_PDF_MB   — max upload size in megabytes (e.g. "50")
 """
-import asyncio
-import json
 import os
 import time
 from copy import deepcopy
-from pathlib import Path
+
+import httpx
 
 from app.core.config import settings
 
+_CONFIG_KEY = "doc_service_config.json"
+
 _DEFAULT_STORAGE_CONFIG: dict = {
     "watch_enabled": False,
     "watch_path": "/data/watch",
@@ -63,33 +64,30 @@ _cache_at: float = 0.0
 _CACHE_TTL = 30.0
 
 
-def _read_config_sync() -> dict:
-    path = Path(settings.CONFIG_PATH)
-    if not path.exists():
-        base = deepcopy(_DEFAULT_CONFIG)
-    else:
-        with open(path) as f:
-            base = json.load(f)
-    return _apply_env_overrides(base)
+def _storage_url() -> str:
+    return f"{settings.STORAGE_SERVICE_URL}/objects/config/{_CONFIG_KEY}"
 
 
-def _read_config_sync_raw() -> dict:
-    """Read without env overrides — used when we need to write back to disk."""
-    path = Path(settings.CONFIG_PATH)
-    if not path.exists():
-        return deepcopy(_DEFAULT_CONFIG)
-    with open(path) as f:
-        return json.load(f)
+async def _fetch_config() -> dict:
+    """Fetch config from storage-service. Returns defaults if not found."""
+    async with httpx.AsyncClient(timeout=10.0) as client:
+        resp = await client.get(_storage_url())
+        if resp.status_code == 404:
+            return deepcopy(_DEFAULT_CONFIG)
+        resp.raise_for_status()
+        return resp.json()
 
 
-def _write_config_sync(config: dict) -> None:
-    """Atomically write config JSON to disk."""
-    path = Path(settings.CONFIG_PATH)
-    tmp = path.with_suffix(".tmp")
-    tmp.parent.mkdir(parents=True, exist_ok=True)
-    with open(tmp, "w") as f:
-        json.dump(config, f, indent=2)
-    os.replace(tmp, path)
+async def _write_config(data: dict) -> None:
+    import json
+    payload = json.dumps(data, indent=2).encode()
+    async with httpx.AsyncClient(timeout=10.0) as client:
+        resp = await client.put(
+            _storage_url(),
+            content=payload,
+            headers={"Content-Type": "application/octet-stream"},
+        )
+        resp.raise_for_status()
 
 
 def _apply_env_overrides(config: dict) -> dict:
@@ -108,7 +106,8 @@ async def load_doc_config() -> dict:
     now = time.monotonic()
     if _cache is not None and (now - _cache_at) < _CACHE_TTL:
         return _cache
-    data = await asyncio.to_thread(_read_config_sync)
+    raw = await _fetch_config()
+    data = _apply_env_overrides(raw)
     _cache = data
     _cache_at = now
     return data
@@ -123,11 +122,10 @@ async def get_storage_config() -> dict:
 
 
 async def save_storage_config(data: dict) -> None:
-    """Merge data into the storage config block and persist to disk."""
+    """Merge data into the storage config block and persist to storage-service."""
     global _cache, _cache_at
-    raw = await asyncio.to_thread(_read_config_sync_raw)
+    raw = await _fetch_config()
     raw.setdefault("storage", {}).update(data)
-    await asyncio.to_thread(_write_config_sync, raw)
-    # Invalidate cache so next read picks up the new values
+    await _write_config(raw)
     _cache = None
     _cache_at = 0.0