fix: image gen result persistence with fallback for temp message IDs, bump v0.17.3

Root cause: assistant messages use temporary IDs (temp-assistant-{timestamp})
in the frontend, which don't match real DB message IDs. The PUT update by
message_id always found 0 rows, so results were never persisted.

Fix — fallback search by session + prompt:
- db.ts: updateMessageContent now returns changes count; new
  updateMessageBySessionAndHint() finds the latest assistant message
  containing image-gen-request with matching prompt text and updates it
- /api/chat/messages PUT: tries message_id first, falls back to
  session_id + prompt_hint search; returns 404 if neither finds a match;
  response includes updated_message_id and fallback_used for debugging
- ImageGenConfirmation: always sends session_id and prompt_hint in PUT
  request, regardless of whether messageId is a real or temp ID

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

src/components/chat/ImageGenConfirmation.tsx

@@ -143,7 +143,7 @@ export function ImageGenConfirmation({
         }
 
         // Persist result to DB by replacing image-gen-request with image-gen-result
-        if (messageId) {
+        {
           const resultBlock = JSON.stringify({
             status: 'completed',
             prompt,
@@ -158,8 +158,10 @@ export function ImageGenConfirmation({
             method: 'PUT',
             headers: { 'Content-Type': 'application/json' },
             body: JSON.stringify({
-              message_id: messageId,
+              message_id: messageId || '',
               content: '```image-gen-result\n' + resultBlock + '\n```',
+              session_id: sessionId,
+              prompt_hint: initialPrompt,
             }),
           }).catch(() => {});
         }