diff --git a/cmd/ingestor/async_migration.go b/cmd/ingestor/async_migration.go
new file mode 100644
index 00000000..7cb014d4
--- /dev/null
+++ b/cmd/ingestor/async_migration.go
@@ -0,0 +1,54 @@
+// Async migration helper — runs schema/backfill work that may take minutes on
+// large prod tables WITHOUT blocking ingestor startup.
+//
+// MIGRATION ANNOTATION CONVENTION (read this before touching migrations):
+//
+//   Sync schema/data migrations (CREATE INDEX, ALTER TABLE, UPDATE ... WHERE)
+//   that run inline during OpenStore() block the ingestor from accepting
+//   packets until they finish. On an empty dev DB they return in milliseconds;
+//   at prod scale (1.9M+ observations, 80K+ adverts) they can pin the boot
+//   for minutes and trigger restart loops. This regression class has bitten us
+//   repeatedly (#791 resolved_path backfill, #1483 obs_observer_ts_idx_v1).
+//
+//   ANY new CREATE INDEX / ALTER TABLE / data-rewrite migration MUST EITHER:
+//     1. Run via Store.RunAsyncMigration(...) below (preferred for backfills
+//        and any work that may touch >1K rows). The migration is recorded as
+//        `pending_async` immediately, returns to the caller (boot proceeds),
+//        and completes in a goroutine. Status flips to `done` (or `failed`
+//        with an error message) when fn returns.
+//     2. Carry the preflight annotation comment immediately above the
+//        migration block, e.g.
+//             // PREFLIGHT: async=true reason="<one-line justification>"
+//        Use this for migrations that are genuinely cheap at any scale
+//        (e.g. ALTER TABLE ADD COLUMN, CREATE INDEX on a known-bounded
+//        table). The annotation is grepped by
+//        ~/.openclaw/skills/pr-preflight/scripts/check-async-migrations.sh
+//        — its absence on a touched migration block is a hard-fail gate.
+//
+//   See MIGRATIONS.md in the repo root for the full policy and examples.
+
+package main
+
+import (
+	"context"
+	"database/sql"
+)
+
+// RunAsyncMigration registers `name` as a pending async migration and
+// schedules `fn` to run in a background goroutine. It returns to the caller
+// immediately so the ingestor can keep booting.
+//
+// STUB: red-commit version. Implementation lands in the green commit; the
+// failing test in async_migration_test.go pins the contract.
+func (s *Store) RunAsyncMigration(ctx context.Context, name string, fn func(context.Context, *sql.DB) error) error {
+	return nil
+}
+
+// AsyncMigrationStatus returns the current status of an async migration
+// (one of "pending_async", "done", "failed") or sql.ErrNoRows if no such
+// migration has been registered.
+//
+// STUB: red-commit version.
+func (s *Store) AsyncMigrationStatus(name string) (string, error) {
+	return "", sql.ErrNoRows
+}
diff --git a/cmd/ingestor/async_migration_test.go b/cmd/ingestor/async_migration_test.go
new file mode 100644
index 00000000..fe982f6c
--- /dev/null
+++ b/cmd/ingestor/async_migration_test.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+	"time"
+)
+
+// TestRunAsyncMigration_PendingThenDone pins the contract for RunAsyncMigration:
+//
+//   1. After calling, the migration name MUST be queryable in the migrations
+//      table with status `pending_async` IMMEDIATELY (no waiting for fn).
+//   2. After fn returns, the status MUST transition to `done`.
+//   3. RunAsyncMigration MUST return without blocking on fn.
+//
+// This is the regression test for the recurring "sync migration on large
+// table blocks ingestor startup" class (#791, #1483, ...). If this test
+// fails the contract is broken — do not relax it; fix the runner.
+func TestRunAsyncMigration_PendingThenDone(t *testing.T) {
+	s := newTestStore(t)
+	ctx := context.Background()
+
+	started := make(chan struct{})
+	release := make(chan struct{})
+
+	const name = "test_async_migration_v1"
+	if err := s.RunAsyncMigration(ctx, name, func(ctx context.Context, db *sql.DB) error {
+		close(started)
+		<-release
+		return nil
+	}); err != nil {
+		t.Fatalf("RunAsyncMigration returned error: %v", err)
+	}
+
+	// Wait for the goroutine to actually start before checking status; this
+	// proves RunAsyncMigration did not block on fn and that fn is running
+	// concurrently.
+	select {
+	case <-started:
+	case <-time.After(2 * time.Second):
+		t.Fatal("async migration fn did not start within 2s — RunAsyncMigration may have blocked or never scheduled")
+	}
+
+	status, err := s.AsyncMigrationStatus(name)
+	if err != nil {
+		t.Fatalf("AsyncMigrationStatus while running: %v", err)
+	}
+	if status != "pending_async" {
+		t.Fatalf("status while fn running: got %q, want %q", status, "pending_async")
+	}
+
+	close(release)
+
+	// Poll for transition to done.
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		status, err = s.AsyncMigrationStatus(name)
+		if err == nil && status == "done" {
+			return
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("status never transitioned to done within 2s: got %q (err=%v)", status, err)
+}