fix: use AppData path for SQLite DB on Windows, absolute path on Linux

The default sqlite:////app/data/cloud.db path only works inside Docker.
On Windows, Controlled Folder Access blocks writes to arbitrary paths.

Now mirrors the same pattern as local_backend/config.py:
  - Windows: %LOCALAPPDATA%/xenia_cloud/cloud.db (always writable)
  - Linux/Docker: next to main.py (overridden by DATABASE_URL in .env)

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

cloud_backend/config.py

@@ -1,12 +1,20 @@
+import os
 from pathlib import Path
 from pydantic_settings import BaseSettings
 
 _HERE = Path(__file__).parent
 
+if os.name == "nt":
+    _DB_DIR = Path(os.environ.get("LOCALAPPDATA", Path.home() / "AppData" / "Local")) / "xenia_cloud"
+    _DB_DIR.mkdir(parents=True, exist_ok=True)
+    _DEFAULT_DB = _DB_DIR / "cloud.db"
+else:
+    _DEFAULT_DB = _HERE / "cloud.db"
+
 
 class Settings(BaseSettings):
     SECRET_KEY: str = "change-me-generate-a-long-random-string"
-    DATABASE_URL: str = "sqlite:////app/data/cloud.db"
+    DATABASE_URL: str = f"sqlite:///{_DEFAULT_DB.as_posix()}"
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 60
     ADMIN_USERNAME: str = "sysadmin"
     ADMIN_PASSWORD: str = "changeme"