Add DB+.env auto backup with download and upload restore in Settings.

Backups land under /root/eth_hedge_backups; restore accepts zip body for new-server migration and restarts the process.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/credentials.py

@@ -20,6 +20,12 @@ def _env_paths() -> list[Path]:
     return [root / ".env", Path.cwd() / ".env", Path.cwd().parent / ".env"]
 
 
+def resolve_env_file_path() -> Path:
+    """第一个已存在的 .env；都不存在则返回仓库根 .env 路径（可写入）。"""
+    paths = _env_paths()
+    return next((p for p in paths if p.is_file()), paths[0])
+
+
 def _ensure_loaded() -> None:
     global _username, _password
     if _username is not None and _password is not None: