fix(backup,tags): unwedge backups on NFS (#739) + tag-standardize "0 groups" (#740)

#739 — DB backups hung on NFS in uninterruptible D-state, defeating the 12-min
subprocess timeout AND Celery's hard limit, so a stuck pg_dump held the
concurrency-1 maintenance_long lane for hours — starving normalize_tags,
re-extract, audits, and the new series rescan (which is why #740 "never
applied"). Three fixes:
- _run_bounded: Popen + bounded post-kill reap; if the child is unkillable
  (D-state) we stop waiting and re-raise TimeoutExpired, freeing the slot. The
  orphan is reaped by the OS once its syscall clears.
- backup_db dumps to a LOCAL temp file then moves the finished .sql to the
  (NFS) _backups dir — pg_dump's long phase is now a DB-socket wait + local
  writes (killable) instead of an NFS write that hangs. backup_images keeps
  bounded-kill (too big to stage locally).
- recover_stalled_backup_runs: split the stall window — db 40 min (was sharing
  images' 7h), so a hung DB backup is flipped to error promptly.

#740 — Standardize tag casing showed "0 groups to change" the instant it was
clicked: onNormCommit overwrote the preview with zeros. Keep the real preview
visible and disable the button while queued; backend apply was already correct.

Tests: fake subprocess.Popen alongside run; bounded-kill fail-fast; local-temp
target; per-kind stall sweep.

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

frontend/src/components/settings/TagMaintenanceCard.vue

@@ -181,7 +181,7 @@
         <v-btn
           color="error" variant="flat" rounded="pill"
           prepend-icon="mdi-format-letter-case"
-          :disabled="!normPreview.total_changes"
+          :disabled="!normPreview.total_changes || normResult === 'queued'"
           :loading="normCommitting"
           @click="onNormCommit"
         >Standardize {{ normPreview.total_changes }} tag group(s)</v-btn>
@@ -291,7 +291,11 @@ async function onNormCommit() {
     // 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: [] }
+    // Keep the preview showing what was QUEUED — do NOT zero it out. Overwriting
+    // it with a zeroed object made the card read "0 tag groups to change" the
+    // instant Standardize was clicked, which looked like nothing happened
+    // (operator-flagged 2026-06-07). The button disables on `queued`; re-running
+    // Preview later reflects the real remaining count as the task applies.
   } finally {
     normCommitting.value = false
   }