feat: add source adapter base class and local filesystem adapter

- Abstract SourceAdapter with FileInfo dataclass
- LocalAdapter for filesystem backups
- Adapter factory for extensibility

backend/backup/adapters/__init__.py

@@ -0,0 +1,13 @@
+from typing import Dict, Any
+from .base import SourceAdapter
+from .local import LocalAdapter
+
+ADAPTER_MAP = {
+    "local": LocalAdapter,
+}
+
+def get_adapter(source_type: str, config: Dict[str, Any]) -> SourceAdapter:
+    adapter_class = ADAPTER_MAP.get(source_type)
+    if not adapter_class:
+        raise ValueError(f"Unknown source type: {source_type}")
+    return adapter_class(config)