Add presigned URL support for direct S3 downloads (#48)

- Add ORCHARD_DOWNLOAD_MODE config (presigned, redirect, proxy)
- Add ORCHARD_PRESIGNED_URL_EXPIRY config (default: 3600s)
- Add generate_presigned_url() method to S3Storage
- Modify download endpoint to support ?mode= query parameter
- Add /url endpoint for getting presigned URL without redirect
- Add PresignedUrlResponse schema with URL, expiry, and checksums
- Default download mode is now presigned for better performance

backend/app/config.py

@@ -32,6 +32,10 @@ class Settings(BaseSettings):
     s3_secret_access_key: str = ""
     s3_use_path_style: bool = True
 
+    # Download settings
+    download_mode: str = "presigned"  # "presigned", "redirect", or "proxy"
+    presigned_url_expiry: int = 3600  # Presigned URL expiry in seconds (default: 1 hour)
+
     @property
     def database_url(self) -> str:
         sslmode = f"?sslmode={self.database_sslmode}" if self.database_sslmode else ""