Add raw AI response to parse result for debugging

- Backend returns raw AI text alongside parsed fields
- Drawer shows expandable "Raw AI response" section when result has empty fields
- Scan Receipt shows raw response inline if no fields were extracted
- Helps diagnose model output issues without needing server logs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/v1/transactions.py

@@ -363,6 +363,7 @@ async def _call_ai_parse(file_bytes: bytes, mime_type: str, user_row) -> dict:
         "date":        parsed.get("date"),
         "description": parsed.get("description"),
         "category":    parsed.get("category"),
+        "raw":         text,
     }