fix(tags): time-box + self-resume the tag standardization (stop the 40-min timeout)

normalize_tags_task timed out at the 40-min hard limit on a large back-catalog
(the first run recases the whole booru vocabulary) — operator-flagged, and it
monopolized the concurrency-1 maintenance queue while doing so.

normalize_existing_tags now takes time_budget_seconds: the live run stops
cleanly at the budget and reports {partial, remaining}. The task runs 600s
chunks and re-enqueues itself until nothing remains (idempotent — commits per
group, so the next chunk skips already-canonical groups). Short chunks let the
recovery sweep and other maintenance tasks interleave instead of being blocked
for 40 minutes.

Frontend: the Standardize button is now fire-and-forget ("Queued — runs in the
background; re-run Preview to confirm") instead of poll-until-done, which would
have falsely reported "complete" after the first chunk.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/settings/TagMaintenanceCard.vue

@@ -185,13 +185,9 @@
           :loading="normCommitting"
           @click="onNormCommit"
         >Standardize {{ normPreview.total_changes }} tag group(s)</v-btn>
-        <span
-          v-if="normResult"
-          class="ml-3 text-caption"
-          :class="normResult === 'ok' ? 'text-success' : 'text-warning'"
-        >
-          <template v-if="normResult === 'ok'">Standardization complete ✓</template>
-          <template v-else>Finished with status: {{ normResult }}</template>
+        <span v-if="normResult === 'queued'" class="ml-3 text-caption text-success">
+          Queued ✓ — runs in the background (you can leave this page). It
+          processes in chunks, so re-run “Preview” later to confirm it's all done.
         </span>
       </div>
     </v-card-text>
@@ -289,13 +285,12 @@ async function onNormCommit() {
   normCommitting.value = true
   normResult.value = null
   try {
-    // Long op (FK repoints): enqueue the maintenance task, then tail the
-    // activity dashboard until its row reaches a terminal status. (The
-    // per-run summary dict isn't exposed by /activity/runs, so we surface
-    // the terminal status — the dry-run preview is the detailed view.)
-    const { task_id: taskId } = await store.normalizeTags({ dryRun: false })
-    const row = await store.pollTaskUntilDone(taskId)
-    normResult.value = row?.status || 'ok'
+    // Fire-and-forget: the task is time-boxed and self-resuming across chunks
+    // (a large back-catalog can't finish in one run), so we DON'T poll-until-
+    // done — that would falsely report "complete" after the first chunk. Just
+    // confirm it's queued; the operator can re-run Preview later to verify.
+    await store.normalizeTags({ dryRun: false })
+    normResult.value = 'queued'
     normPreview.value = { total_changes: 0, tags_to_rename: 0, collisions: 0, tags_to_merge: 0, sample: [] }
   } finally {
     normCommitting.value = false