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/base.py

@@ -0,0 +1,39 @@
+from abc import ABC, abstractmethod
+from typing import List, Dict, Any, AsyncIterator
+from dataclasses import dataclass
+
+@dataclass
+class FileInfo:
+    path: str
+    size: int
+    modified_time: float
+    is_directory: bool
+
+class SourceAdapter(ABC):
+    def __init__(self, config: Dict[str, Any]):
+        self.config = config
+    
+    @abstractmethod
+    async def connect(self) -> None:
+        """Establish connection to source."""
+        pass
+    
+    @abstractmethod
+    async def disconnect(self) -> None:
+        """Close connection to source."""
+        pass
+    
+    @abstractmethod
+    async def list_files(self, path: str = "") -> List[FileInfo]:
+        """List files at given path."""
+        pass
+    
+    @abstractmethod
+    async def read_file(self, path: str) -> AsyncIterator[bytes]:
+        """Read file in chunks."""
+        pass
+    
+    @abstractmethod
+    async def get_database_dump(self, config: Dict[str, Any]) -> AsyncIterator[bytes]:
+        """Get database dump. Only implemented for database adapters."""
+        pass