fc5: migration 0015 + MigrationRun model (kind/status as String(32), API-validated)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -10,6 +10,7 @@ from .image_record import ImageRecord
 from .import_batch import ImportBatch
 from .import_settings import ImportSettings
 from .import_task import ImportTask
+from .migration_run import MigrationRun
 from .ml_settings import MLSettings
 from .post import Post
 from .post_attachment import PostAttachment
@@ -40,6 +41,7 @@ __all__ = [
     "ImportTask",
     "ImportSettings",
     "MLSettings",
+    "MigrationRun",
     "TagAlias",
     "TagAllowlist",
     "TagReferenceEmbedding",