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/api/__init__.py

@@ -26,7 +26,6 @@ def all_blueprints() -> list[Blueprint]:
     from .extension import extension_bp
     from .gallery import gallery_bp
     from .import_admin import import_admin_bp
-    from .migrate import migrate_bp
     from .ml_admin import ml_admin_bp
     from .platforms import platforms_bp
     from .posts import posts_bp
@@ -54,7 +53,6 @@ def all_blueprints() -> list[Blueprint]:
         admin_bp,
         cleanup_bp,
         import_admin_bp,
-        migrate_bp,
         suggestions_bp,
         allowlist_bp,
         aliases_bp,