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>

tests/test_tag_normalize.py

@@ -123,6 +123,29 @@ async def test_live_merges_case_variants_and_repoints_images(db):
     assert assoc == 3
 
 
+@pytest.mark.asyncio
+async def test_time_budget_stops_partial_and_reports_remaining(db):
+    """A zero budget stops before the first group so a huge back-catalog can't
+    run the task into the Celery time limit; it reports partial/remaining so the
+    caller re-enqueues to continue (operator-flagged 40-min timeout 2026-06-07)."""
+    await _raw_tag(db, "alpha", TagKind.general)
+    await _raw_tag(db, "beta", TagKind.general)
+
+    summary = await normalize_existing_tags(db, dry_run=False, time_budget_seconds=0)
+
+    assert summary["partial"] is True
+    assert summary["groups_processed"] == 0
+    assert summary["remaining"] == summary["total_changes"] >= 2
+    # Nothing recased yet — the budget cut before any work.
+    assert await _count_named(db, "alpha", TagKind.general) == 1
+
+    # A full (unbudgeted) run finishes and reports nothing remaining.
+    done = await normalize_existing_tags(db, dry_run=False)
+    assert done["partial"] is False
+    assert done["remaining"] == 0
+    assert await _count_named(db, "Alpha", TagKind.general) == 1
+
+
 @pytest.mark.asyncio
 async def test_idempotent_second_run_is_noop(db):
     await _raw_tag(db, "kafka", TagKind.character)