Clean up presigned URL implementation: remove dead fields, fix error handling

- Remove unused image_url from upload response and TS type
- Remove bare except in serve_image that masked config errors as 404s
- Add error state and broken-image placeholder in QuestionBubble

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

raggr-frontend/src/api/conversationService.ts

@@ -125,7 +125,7 @@ class ConversationService {
   async uploadImage(
     file: File,
     conversationId: string,
-  ): Promise<{ image_key: string; image_url: string }> {
+  ): Promise<{ image_key: string }> {
     const formData = new FormData();
     formData.append("file", file);
     formData.append("conversation_id", conversationId);