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/celery_app.py

@@ -28,7 +28,6 @@ def make_celery() -> Celery:
             "backend.app.tasks.import_file",
             "backend.app.tasks.thumbnail",
             "backend.app.tasks.maintenance",
-            "backend.app.tasks.migration",
             "backend.app.tasks.ml",
             "backend.app.tasks.download",
             "backend.app.tasks.backup",
@@ -45,7 +44,6 @@ def make_celery() -> Celery:
             "backend.app.tasks.download.*": {"queue": "download"},
             "backend.app.tasks.scan.*": {"queue": "scan"},
             "backend.app.tasks.maintenance.*": {"queue": "maintenance"},
-            "backend.app.tasks.migration.*": {"queue": "maintenance"},
             "backend.app.tasks.backup.*": {"queue": "maintenance"},
             "backend.app.tasks.admin.*": {"queue": "maintenance"},
             "backend.app.tasks.library_audit.*": {"queue": "maintenance"},