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/models/__init__.py

@@ -12,7 +12,6 @@ from .import_batch import ImportBatch
 from .import_settings import ImportSettings
 from .import_task import ImportTask
 from .library_audit_run import LibraryAuditRun
-from .migration_run import MigrationRun
 from .ml_settings import MLSettings
 from .post import Post
 from .post_attachment import PostAttachment
@@ -46,7 +45,6 @@ __all__ = [
     "ImportSettings",
     "LibraryAuditRun",
     "MLSettings",
-    "MigrationRun",
     "TagAlias",
     "TagAllowlist",
     "TagReferenceEmbedding",

backend/app/models/migration_run.py

@@ -1,37 +0,0 @@
-"""MigrationRun — tracks each FC-5 migration invocation (backup/gs/ir/etc).
-
-kind/status are String(32) not Postgres ENUM so adding kinds later
-doesn't need a schema migration. The API layer validates values.
-"""
-
-from datetime import datetime
-
-import sqlalchemy as sa
-from sqlalchemy import Boolean, DateTime, Integer, String, Text, func
-from sqlalchemy.dialects.postgresql import JSONB
-from sqlalchemy.orm import Mapped, mapped_column
-
-from .base import Base
-
-
-class MigrationRun(Base):
-    __tablename__ = "migration_run"
-
-    id: Mapped[int] = mapped_column(Integer, primary_key=True)
-    kind: Mapped[str] = mapped_column(String(32), nullable=False, index=True)
-    status: Mapped[str] = mapped_column(String(32), nullable=False, index=True)
-    dry_run: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
-    started_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), nullable=False, server_default=func.now(),
-    )
-    finished_at: Mapped[datetime | None] = mapped_column(
-        DateTime(timezone=True), nullable=True,
-    )
-    counts: Mapped[dict] = mapped_column(
-        JSONB, nullable=False, default=dict, server_default=sa.text("'{}'::jsonb"),
-    )
-    error: Mapped[str | None] = mapped_column(Text, nullable=True)
-    metadata_: Mapped[dict] = mapped_column(
-        "metadata", JSONB, nullable=False, default=dict,
-        server_default=sa.text("'{}'::jsonb"),
-    )