feat(api): GET /graphs/{slug}/migrations endpoint (Phase 7 #59)

[charlie83Gs]

Summary

Ships the read-only half of the migration-audit API. Returns rows from `graph_migration_runs` for the graph, newest-first, as `GraphMigrationRunResponse`. Backs the Phase 7 frontend history table (#61) and gives operators a debug path without needing Hatchet UI access.

Split from the companion `POST /graphs/{slug}/re-migrate` (also part of #59) because re-migrate needs `graph_migration_wf` to exist (#57) to dispatch against; this list endpoint only depends on the audit table (#6, already shipped).

Contract

• `GET /api/v1/graphs/{slug}/migrations`
• Requires `GRAPH_READ` — audit rows can imply when data transformations touched the graph.
• Empty list when graph has never migrated (brand-new graphs at v1, any graph on a plugin still at `current_version=1`).
• Ordered by `created_at` desc so the UI can render "most recent first" without client-side sorting.
• Read-only banner on the UI continues to rely on `Graph.read_only_reason='migrating'` — this endpoint is for history + debug only, not the banner's source of truth.

Schema design

`status` is `str`, not `Literal`. The DB column is `VARCHAR(16)` with a CHECK constraint at the model layer; validation lives there. New states added later (e.g. `'cancelled'`) won't require a coordinated API + frontend ship.

Test plan

• 3 new tests in `test_graph_schemas.py::TestGraphMigrationRunResponse`: minimum-field construction, full round-trip including error/workflow_run_id, status stays open-ended
• `services/api`: 83 unit tests pass
• `ruff format` + pre-push hooks clean

🤖 Generated with Claude Code

[charlie83Gs]

Review fixes applied

• Tie-break on `id.desc()` — batch migrations enqueueing multiple pending rows in the same tick now sort deterministically. Load-bearing for future cursor pagination.
• `?limit` query param — clamps response size. Default 100, server-side max 500 (values outside return 422). Keeps long-lived graphs on many-version plugins from returning unbounded audit history.
• 8 new HTTP-level integration tests in `test_migrations_list_endpoint.py`: 404 on unknown slug, empty list for never-migrated graph, newest-first ordering, id-desc tie-break, cross-graph scoping (rows don't leak), limit clamps, limit=0 → 422, limit>max → 422. Mirrors fixture pattern from `test_graph_read_only_endpoint.py`.
• Status comment dropped — docstring now points at `kt_db.models.GraphMigrationRun` for the authoritative status set + DB CHECK, so the enum definition doesn't drift between sites.
• datetime imports — moved to module level; the `import("datetime").datetime.now()` hack is gone.

Kept `graph_id` in the response: dropping it is technically payload-smaller but some frontend list consumers key by it when rendering multiple graphs' history side-by-side. Low-value drop; parking until a consumer actually needs the savings.

Error-field scrub (re: internal paths/traceback leaks) remains a worker (#57) concern per your note.

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