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

backend/app/schemas.py

@@ -330,6 +330,20 @@ class GlobalSearchResponse(BaseModel):
     counts: Dict[str, int]  # Total counts for each type
 
 
+# Presigned URL response
+class PresignedUrlResponse(BaseModel):
+    """Response containing a presigned URL for direct S3 download"""
+    url: str
+    expires_at: datetime
+    method: str = "GET"
+    artifact_id: str
+    size: int
+    content_type: Optional[str] = None
+    original_name: Optional[str] = None
+    checksum_sha256: Optional[str] = None
+    checksum_md5: Optional[str] = None
+
+
 # Health check
 class HealthResponse(BaseModel):
     status: str