ui(sidebar): drop Title field, add bulk notes editor

The Title (user_title) field hadn't earned its place in the sidebar
form — the underlying column stays on the model but the editable
row + its draft state + commit handler are gone.

Bulk Notes: a textarea in the multi-photo bulk panel that replaces
user_notes across the whole selection with one string. Apply commits;
Clear empties the draft without committing. New backend bulk action
'set_notes' validates the value is a string (or null/empty to clear)
and writes to every photo in the selection in one go. Wired through
the standard useBulkPhotoMutations optimistic-patch path, so the
photo cache flips immediately and rolls back on error.

user_notes added to the shared Photo type so patchPhotos accepts the
field; previously it was only on PhotoInfoPanel's local PhotoDetails.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useBulkPhotoMutations.ts

@@ -172,5 +172,22 @@ export function useBulkPhotoMutations() {
     onSuccess: invalidatePhotoQueries,
   })
 
-  return { bulkRating, bulkColor, invalidatePhotoQueries }
+  const bulkNotes = useMutation({
+    mutationFn: ({ ids, notes }: { ids: string[]; notes: string }) =>
+      photosApi.bulkSetNotes(ids, notes),
+    onMutate: ({ ids, notes }) => {
+      const snapshot = snapshotPhotos(ids)
+      // Mirror the server's empty-string -> null collapse so the
+      // optimistic cache matches what the API returns.
+      patchPhotos(ids, { user_notes: notes || null })
+      return { snapshot }
+    },
+    onError: (e, _vars, ctx) => {
+      if (ctx?.snapshot) restoreFromSnapshot(ctx.snapshot)
+      toast.error('Notes update failed', formatApiError(e))
+    },
+    onSuccess: invalidatePhotoQueries,
+  })
+
+  return { bulkRating, bulkColor, bulkNotes, invalidatePhotoQueries }
 }