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,

backend/app/api/migrate.py

@@ -1,107 +0,0 @@
-"""FC-5: /api/migrate — trigger and poll migration runs.
-
-Ingest kinds (gs_ingest, ir_ingest) accept multipart/form-data with an
-`export_file` field. All other kinds accept JSON. Backup + rollback
-were retired in FC-3h (2026-05-24); use /api/system/backup/* instead.
-"""
-
-import json
-
-from quart import Blueprint, jsonify, request
-from sqlalchemy import select
-
-from ..extensions import get_session
-from ..models import MigrationRun
-from ..tasks.migration import run_migration
-from ._responses import error_response as _bad
-
-migrate_bp = Blueprint("migrate", __name__, url_prefix="/api/migrate")
-
-# 'backup' + 'rollback' retired 2026-05-24 (FC-3h); see /api/system/backup/*.
-_VALID_KINDS = frozenset({
-    "gs_ingest", "ir_ingest", "tag_apply",
-    "ml_queue", "verify", "cleanup",
-})
-_INGEST_KINDS = frozenset({"gs_ingest", "ir_ingest"})
-
-
-def _run_to_dict(run: MigrationRun) -> dict:
-    return {
-        "id": run.id,
-        "kind": run.kind,
-        "status": run.status,
-        "dry_run": run.dry_run,
-        "started_at": run.started_at.isoformat(),
-        "finished_at": run.finished_at.isoformat() if run.finished_at else None,
-        "counts": run.counts or {},
-        "error": run.error,
-        "metadata": run.metadata_ or {},
-    }
-
-
-@migrate_bp.route("/<kind>", methods=["POST"])
-async def create_run(kind: str):
-    if kind not in _VALID_KINDS:
-        return _bad("unknown_kind", detail=f"kind must be one of {sorted(_VALID_KINDS)}")
-
-    # Ingest kinds accept multipart/form-data; everything else takes JSON.
-    if kind in _INGEST_KINDS:
-        form = await request.form
-        files = await request.files
-        if "export_file" not in files:
-            return _bad("missing_export_file", detail="multipart export_file required")
-        export_file = files["export_file"]
-        try:
-            raw = export_file.read()
-            data = json.loads(raw.decode("utf-8"))
-        except (UnicodeDecodeError, json.JSONDecodeError) as exc:
-            return _bad("invalid_export_file", detail=str(exc))
-        dry_run = str(form.get("dry_run", "false")).lower() in ("true", "1", "yes")
-        params: dict = {"data": data, "dry_run": dry_run}
-    else:
-        body = await request.get_json()
-        if body is None:
-            body = {}
-        if not isinstance(body, dict):
-            return _bad("invalid_body")
-        dry_run = bool(body.get("dry_run", False))
-        params = dict(body)
-
-    async with get_session() as session:
-        run = MigrationRun(kind=kind, status="pending", dry_run=dry_run)
-        session.add(run)
-        await session.commit()
-        await session.refresh(run)
-        run_id = run.id
-
-    run_migration.delay(run_id, kind, params)
-    return jsonify({"run_id": run_id, "status": "pending"}), 202
-
-
-@migrate_bp.route("/runs/<int:run_id>", methods=["GET"])
-async def get_run(run_id: int):
-    async with get_session() as session:
-        run = (await session.execute(
-            select(MigrationRun).where(MigrationRun.id == run_id)
-        )).scalar_one_or_none()
-    if run is None:
-        return _bad("not_found", status=404)
-    return jsonify(_run_to_dict(run))
-
-
-@migrate_bp.route("/runs", methods=["GET"])
-async def list_runs():
-    try:
-        limit = int(request.args.get("limit", "10"))
-    except ValueError:
-        return _bad("invalid_limit")
-    if limit < 1 or limit > 100:
-        return _bad("invalid_limit")
-
-    async with get_session() as session:
-        rows = (await session.execute(
-            select(MigrationRun)
-            .order_by(MigrationRun.id.desc())
-            .limit(limit)
-        )).scalars().all()
-    return jsonify([_run_to_dict(r) for r in rows])