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>

alembic/versions/0027_drop_migration_run.py

@@ -0,0 +1,50 @@
+"""drop migration_run — one-and-done GS/IR migration tooling removed
+
+Revision ID: 0027
+Revises: 0026
+Create Date: 2026-05-29
+
+The GS/IR migration tooling (services/migrators, /api/migrate, the
+run_migration task, LegacyMigrationCard, and the MigrationRun model) was
+removed after the migration cutover completed. This drops its now-orphaned
+run-log table. Downgrade recreates the table (mirrors the old model) so the
+migration is reversible.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import JSONB
+
+revision: str = "0027"
+down_revision: Union[str, None] = "0026"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_table("migration_run")
+
+
+def downgrade() -> None:
+    op.create_table(
+        "migration_run",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column("kind", sa.String(length=32), nullable=False),
+        sa.Column("status", sa.String(length=32), nullable=False),
+        sa.Column("dry_run", sa.Boolean(), nullable=False, server_default=sa.false()),
+        sa.Column(
+            "started_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+        sa.Column("finished_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column(
+            "counts", JSONB(), nullable=False, server_default=sa.text("'{}'::jsonb"),
+        ),
+        sa.Column("error", sa.Text(), nullable=True),
+        sa.Column(
+            "metadata", JSONB(), nullable=False, server_default=sa.text("'{}'::jsonb"),
+        ),
+    )
+    op.create_index("ix_migration_run_kind", "migration_run", ["kind"])
+    op.create_index("ix_migration_run_status", "migration_run", ["status"])