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/api/admin.py

@@ -245,6 +245,32 @@ async def tags_reset_content():
     return jsonify(result)
 
 
+@admin_bp.route("/tags/normalize", methods=["POST"])
+async def tags_normalize():
+    """#714: retro-normalize existing tags to the #701 canonical form (Title
+    Case + collapsed whitespace) and merge case/whitespace-variant duplicates.
+
+    dry_run=true (default) returns a projection inline — group/collision/rename
+    counts + a sample of the changes — so the UI shows exactly what'll happen.
+    dry_run=false dispatches the long-running maintenance task (the merge FK
+    repoints can touch many tags); the UI tails the activity dashboard for the
+    summary. Idempotent; back up first (the merges are irreversible)."""
+    from ..services.tag_service import normalize_existing_tags
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", True))
+
+    if dry_run:
+        async with get_session() as session:
+            result = await normalize_existing_tags(session, dry_run=True)
+        return jsonify(result)
+
+    from ..tasks.admin import normalize_tags_task
+
+    async_result = normalize_tags_task.delay()
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
 @admin_bp.route("/maintenance/db-stats", methods=["GET"])
 async def db_stats():
     """Per-table bloat readout (pg_stat_user_tables) for the high-churn tables