Add local storage provider implementation

Introduce an abstract `StorageProvider` class and a

backend/app/core/storage.py

@@ -0,0 +1,91 @@
+import os
+import shutil
+from abc import ABC, abstractmethod
+from pathlib import Path
+from typing import Tuple
+
+from fastapi import UploadFile
+
+from app.core.config import settings
+
+
+class StorageProvider(ABC):
+    """Base abstract class for storage providers."""
+
+    @abstractmethod
+    async def save_file(self, file: UploadFile, destination: str) -> str:
+        """Save a file to storage and return the relative path."""
+        pass
+
+    @abstractmethod
+    def generate_presigned_url(self, file_path: str,
+                               filename: str,
+                               content_type: str,
+                               expires_in: int = 300) -> Tuple[str, str]:
+        """
+        Generate a presigned URL for file upload.
+        Returns: (upload_url, file_url)
+        """
+        pass
+
+    @abstractmethod
+    def get_file_url(self, file_path: str) -> str:
+        """Get the URL for accessing a file."""
+        pass
+
+
+class LocalStorageProvider(StorageProvider):
+    """Local filesystem storage provider."""
+
+    def __init__(self, base_dir: str, files_url_path: str):
+        self.base_dir = Path(base_dir)
+        self.files_url_path = files_url_path
+
+        # Ensure base directory exists
+        os.makedirs(self.base_dir, exist_ok=True)
+
+    async def save_file(self, file: UploadFile, destination: str) -> str:
+        """Save an uploaded file to the local filesystem."""
+        # Ensure destination directory exists
+        dest_path = self.base_dir / destination
+        os.makedirs(dest_path.parent, exist_ok=True)
+
+        # Save the file
+        with open(dest_path, "wb") as buffer:
+            shutil.copyfileobj(file.file, buffer)
+
+        # Return the relative path
+        return destination
+
+    def generate_presigned_url(self, file_path: str,
+                               filename: str,
+                               content_type: str,
+                               expires_in: int = 300) -> Tuple[str, str]:
+        """
+        Generate a token-based upload URL for local storage.
+        This simulates presigned URLs for local storage.
+        """
+        from app.utils.security import create_upload_token
+
+        # Generate a unique token for this upload
+        token = create_upload_token(file_path, content_type, expires_in)
+
+        # The upload URL is to our custom upload endpoint
+        upload_url = f"{settings.API_VERSION_STR}/uploads/{token}"
+
+        # The file URL is where the file will be accessible after upload
+        file_url = f"{self.files_url_path}/{file_path}"
+
+        return upload_url, file_url
+
+    def get_file_url(self, file_path: str) -> str:
+        """Get the URL for accessing a file."""
+        return f"{self.files_url_path}/{file_path}"
+
+
+def get_storage_provider() -> StorageProvider:
+    """Dependency for getting the configured storage provider."""
+    return LocalStorageProvider(
+        base_dir=settings.DATA_FILES_DIR,
+        files_url_path="/files"
+    )