fix: address all 6 P2 review issues from PR #8 round 4

- BackgroundTasks: per-task Retry now dispatches to the correct backend —
  retryFailedEmbeddings for embedding failures, queueTaggingJobs for tagging
  failures, or both; previously always called embedding retry only

- commands: tag-cloud cache key now hashes both image IDs and embedding bytes
  (xxh3) so re-embedding a file after reindex correctly invalidates the cache;
  removed now-dead fnv_hash_ids helper

- db: update_folder_path uses SUBSTR instead of replace() to rewrite only the
  leading path prefix, preventing corruption when the old folder name also
  appears in a subdirectory name

- db: add_user_tag promotes an existing AI-owned tag to user-owned on conflict
  instead of DO NOTHING, preventing the tagger from deleting a manually added
  tag that coincides with an AI tag

- db/commands: add clear_duplicate_scan_cache / invalidate_duplicate_scan_cache
  so deletion removes the stale persisted cache entry; without this a restart
  would reload groups containing deleted images

- store: setSimilarScope re-runs loadSimilarByRegion (with saved crop) when the
  active collection is Region Search Results, instead of silently switching to
  whole-image similarity and discarding the crop

src/components/BackgroundTasks.tsx

@@ -57,6 +57,7 @@ export function BackgroundTasks() {
   const indexingProgress = useGalleryStore((state) => state.indexingProgress);
   const mediaJobProgress = useGalleryStore((state) => state.mediaJobProgress);
   const retryFailedEmbeddings = useGalleryStore((state) => state.retryFailedEmbeddings);
+  const queueTaggingJobs = useGalleryStore((state) => state.queueTaggingJobs);
   const duplicateScanning = useGalleryStore((state) => state.duplicateScanning);
   const duplicateScanProgress = useGalleryStore((state) => state.duplicateScanProgress);
   const [expanded, setExpanded] = useState(false);
@@ -475,7 +476,10 @@ export function BackgroundTasks() {
                   {taskHasFailed && (
                     <button
                       className="rounded-lg border border-amber-500/20 bg-amber-500/10 px-2 py-0.5 text-[11px] text-amber-300 hover:bg-amber-500/20 transition-colors shrink-0"
-                      onClick={() => void retryFailedEmbeddings(task.id)}
+                      onClick={() => {
+                        if (task.hasFailedEmbeddings) void retryFailedEmbeddings(task.id);
+                        if (task.hasFailedTagging) void queueTaggingJobs(task.id);
+                      }}
                     >
                       Retry
                     </button>