fix: persist created_at in cache so Joplin note titles get date prefix

mark_exported() was discarding created_at from the metadata dict because
it wasn't in the hardcoded stored-key list, so the joplin sync always
saw an empty date and omitted the prefix.

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

src/cache.py

@@ -87,6 +87,7 @@ class Cache:
         self._data[provider][conv_id] = {
             "title": metadata.get("title", ""),
             "project": metadata.get("project"),
+            "created_at": metadata.get("created_at", ""),
             "updated_at": metadata.get("updated_at", ""),
             "exported_at": datetime.now(tz=timezone.utc).isoformat(),
             "file_path": metadata.get("file_path", ""),