refactor(I5): remove one-and-done GS/IR migration tooling

The GS/IR migration cutover is complete, so the runbook tooling is dead
weight. Removed:
- services/migrators/ (gs_ingest, ir_ingest, tag_apply, ml_queue, verify,
  cleanup), tasks/migration.py, api/migrate.py (+ blueprint registration)
- MigrationRun model; alembic 0027 drops the migration_run table
- frontend LegacyMigrationCard + migration store (+ MaintenancePanel ref)
- celery include + task route + celery_signals queue mapping for migration.*
- the 1 GB MAX_CONTENT_LENGTH / MAX_FORM_MEMORY override (added solely for
  the ir_ingest upload)
- migration-surface tests (test_api_migrate, test_migration_verify,
  test_ir_ingest, test_gs_ingest, test_tag_apply)

Kept: the alembic schema-migration tests (test_migration_00XX — unrelated)
and cleanup_service.py (the permanent artist-cascade/unlink home).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/__init__.py

@@ -33,12 +33,6 @@ def create_app() -> Quart:
 
     app = Quart(__name__)
     app.secret_key = cfg.secret_key
-    # FC-5: legacy IR ingest JSON can run to tens of MB (hundreds of
-    # thousands of image_tag_associations). Werkzeug's default form
-    # memory cap is 500KB; raise both ceilings so the multipart upload
-    # for /api/migrate/ir_ingest doesn't 413.
-    app.config["MAX_CONTENT_LENGTH"] = 1024 * 1024 * 1024   # 1 GB
-    app.config["MAX_FORM_MEMORY_SIZE"] = 1024 * 1024 * 1024  # 1 GB
 
     for bp in all_blueprints():
         app.register_blueprint(bp)