feat(tags): retro-normalize existing tags to Title Case + merge case-collisions (plan #714)

Follow-up to #701: new tags are saved canonical, but the back-catalog keeps
whatever casing it was created with. This adds a maintenance action that
Title-Cases every existing tag (collapsing whitespace) and merges
case/whitespace-variant duplicates into one.

Backend:
- tag_service.normalize_existing_tags(session, *, dry_run): groups all tags by
  (kind, coalesce(fandom_id,-1), canonical_name). Per group it picks a survivor
  (prefer an already-canonical member → no rename/self-alias; else the
  best-connected tag → fewest FK repoints; else lowest id), merges the variants
  INTO it via the tested TagService._do_merge (image_tag/allowlist/embedding/
  aliases/series_page repoints + protective ML aliases), then renames the
  survivor to canonical. Losers are deleted before the rename so there's no
  transient unique-index clash; commits per group and isolates failures per
  group. Idempotent — an already-canonical lone tag is a no-op.
- normalize_tags_task (maintenance queue, asyncio.run + per-task NullPool async
  engine, soft 1800/hard 2400) — recovery/timeout/duration covered by FC-3i.
- POST /api/admin/tags/normalize: dry_run=true returns a projection inline
  (group/collision/rename counts + sample); dry_run=false enqueues the task.

Frontend: a "Standardize tag casing" section in TagMaintenanceCard (Cleanup
tab) — preview → apply (polls the activity dashboard to terminal status),
behind a back-up-first warning. admin store gains normalizeTags().

Tests: tests/test_tag_normalize.py — dry-run counts, live merge + image-tag
dedup/repoint, idempotency, same-name-different-fandom and -different-kind kept
separate, ML-known loser keeps a protective alias.

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

backend/app/tasks/admin.py

@@ -73,3 +73,32 @@ def reextract_archive_attachments_task(self) -> dict:
         return cleanup_service.reextract_archive_attachments(
             session, images_root=IMAGES_ROOT,
         )
+
+
+@celery.task(
+    name="backend.app.tasks.admin.normalize_tags_task",
+    bind=True,
+    autoretry_for=(OperationalError, DBAPIError),
+    retry_backoff=15, retry_backoff_max=180, max_retries=1,
+    soft_time_limit=1800, time_limit=2400,  # 30 min / 40 min
+)
+def normalize_tags_task(self) -> dict:
+    """Wraps tag_service.normalize_existing_tags (#714): Title-Case the
+    back-catalog and merge case/whitespace-variant duplicate tags via the
+    tested async merge path. Runs under its own asyncio loop + per-task async
+    engine (NullPool, disposed when the loop ends), mirroring download_source."""
+    import asyncio
+
+    from ..services.tag_service import normalize_existing_tags
+    from ._async_session import async_session_factory
+
+    async def _run() -> dict:
+        async_factory, async_engine = async_session_factory()
+        try:
+            async with async_factory() as session:
+                # normalize_existing_tags commits per group internally.
+                return await normalize_existing_tags(session, dry_run=False)
+        finally:
+            await async_engine.dispose()
+
+    return asyncio.run(_run())