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/services/cleanup_service.py

@@ -6,9 +6,8 @@ HTTP handlers (small ops) and from Celery tasks in
 backend.app.tasks.admin (long ops).
 
 This module is the PERMANENT home of artist-cascade + image-unlink
-logic. The legacy copy at backend/app/services/migrators/cleanup.py
-stays in place until FC-3j; FC-3j will replace its body with thin
-re-exports from this module and then delete the wrapper.
+logic. (The legacy migrators/cleanup.py copy was removed with the rest of
+the one-and-done GS/IR migration tooling.)
 """
 from __future__ import annotations