fix: CRITICAL - Replace S3 URIs with presigned URLs in all API responses

- Changed file_url in session.file_metadata to use presigned_url instead of s3_uri
- Updated file_data in upload messages to use presigned_url
- Fixed fileUrl in message metadata to use presigned_url
- All user-facing fileUrl fields now return accessible presigned URLs
- S3 URIs kept internally as s3_uri field for reference
- Fixes issue where users received unusable s3:// URIs

api_routes_v2.py

@@ -781,7 +781,8 @@ def upload_stream_to_s3(chat_id: str, file: UploadFile) -> str:
         "file_metadata": {
             "has_file": True,
             "file_name": file.filename,
-            "file_url": s3_uri,
+            "file_url": presigned["presigned_url"],  # CRITICAL: Use presigned URL, not S3 URI
+            "s3_uri": s3_uri,  # Keep for internal reference
             "uploaded_at": datetime.utcnow().isoformat() + "Z",
             **presigned  # CHANGE
         }
@@ -795,7 +796,7 @@ def upload_stream_to_s3(chat_id: str, file: UploadFile) -> str:
         "file_data": {
             "has_file": True,
             "file_name": file.filename,
-            "file_url": s3_uri
+            "file_url": presigned["presigned_url"]  # Use presigned URL for user access
         }
     }
 
@@ -1053,8 +1054,8 @@ async def chat_unified(
     if file_info:
         file_meta_for_msg = {
             "fileName": file.filename if file else None,
-            "fileUrl": file_info.get("s3_uri"),
-            "s3_uri": file_info.get("s3_uri")
+            "fileUrl": file_info.get("presigned_url"),  # Use presigned URL, not S3 URI
+            "s3_uri": file_info.get("s3_uri")  # Keep for internal reference
         }
     _add_and_mirror_message(chat_id, "user", message, file_metadata=file_meta_for_msg)
     # CHANGE: maybe generate a session title after first real user message