fix(api): initialize file_path before try block (#497)

This PR fixes a potential UnboundLocalError in the API router
by ensuring file_path is initialized before the try block.

## Issue

In the create_source endpoint, file_path was initialized inside the
try block. If an exception occurred before this initialization,
exception handlers that reference file_path would crash with
UnboundLocalError.

## Fix

- Initialize file_path = None before the try block (line 289)
- Add explanatory comment for future maintainers
- Remove duplicate initialization inside the try block

This ensures exception handlers on line 415 can safely reference
file_path without causing runtime errors.

## Testing

- Verified exception handler path no longer crashes
- Confirmed file cleanup works correctly in error cases

Co-authored-by: POWERFULMOVES <POWERFULMOVES@users.noreply.github.com>
Co-authored-by: Claude Code <noreply@anthropic.com>

api/routers/sources.py

@@ -285,6 +285,9 @@ async def create_source(
     """Create a new source with support for both JSON and multipart form data."""
     source_data, upload_file = form_data
 
+    # Initialize file_path before try block so exception handlers can reference it
+    file_path = None
+
     try:
         # Verify all specified notebooks exist (backward compatibility support)
         for notebook_id in source_data.notebooks or []:
@@ -295,7 +298,6 @@ async def create_source(
                 )
 
         # Handle file upload if provided
-        file_path = None
         if upload_file and source_data.type == "upload":
             try:
                 file_path = await save_uploaded_file(upload_file)