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/local.py

@@ -0,0 +1,47 @@
+import os
+import aiofiles
+from pathlib import Path
+from typing import List, AsyncIterator, Dict, Any
+from .base import SourceAdapter, FileInfo
+
+class LocalAdapter(SourceAdapter):
+    async def connect(self) -> None:
+        base_path = self.config.get("path", ".")
+        if not os.path.exists(base_path):
+            raise FileNotFoundError(f"Path not found: {base_path}")
+    
+    async def disconnect(self) -> None:
+        pass
+    
+    async def list_files(self, path: str = "") -> List[FileInfo]:
+        base_path = Path(self.config.get("path", "."))
+        target_path = base_path / path if path else base_path
+        
+        files = []
+        exclude_patterns = self.config.get("exclude", [])
+        
+        for item in target_path.iterdir():
+            # Check exclude patterns
+            if any(item.match(pattern) for pattern in exclude_patterns):
+                continue
+            
+            stat = item.stat()
+            files.append(FileInfo(
+                path=str(item.relative_to(base_path)),
+                size=stat.st_size,
+                modified_time=stat.st_mtime,
+                is_directory=item.is_dir()
+            ))
+        
+        return files
+    
+    async def read_file(self, path: str) -> AsyncIterator[bytes]:
+        base_path = Path(self.config.get("path", "."))
+        file_path = base_path / path
+        
+        async with aiofiles.open(file_path, "rb") as f:
+            while chunk := await f.read(8192):
+                yield chunk
+    
+    async def get_database_dump(self, config: Dict[str, Any]) -> AsyncIterator[bytes]:
+        raise NotImplementedError("Local adapter does not support database dumps")