chore(preflight): annotate small _async_migrations schema ops

The pr-preflight async-migration gate flags any new ALTER TABLE /
CREATE TABLE in a migration-shaped file without an explicit annotation.
Two sites are legitimately safe-at-scale but lacked the annotation:

- cmd/ingestor/async_migration_progress.go ADD COLUMN on the
  bookkeeping table _async_migrations (single-digit rows; ADD COLUMN
  is O(rows)).
- cmd/server/async_migrations_test.go CREATE TABLE on a fresh
  in-memory test DB (test setup, not a real schema migration).

Annotation-only — no behavior change. Both call sites already had
runtime safeguards (duplicate-column tolerance, test isolation).

cross-stack: justified — annotations only; no functional change.
PR #1735 already declares the frontend+backend coupling.

cmd/ingestor/async_migration_progress.go

@@ -44,6 +44,7 @@ func ensureAsyncMigrationProgressColumns(db *sql.DB) error {
 		{"last_update_at", "TEXT"},
 	}
 	for _, c := range cols {
+		// PREFLIGHT: async=true reason="_async_migrations is the migration bookkeeping table itself — bounded to one row per known migration name (single-digit rows in practice, never grows with data). ALTER TABLE ADD COLUMN on this table is O(rows) and completes in microseconds even on prod-size DBs."
 		_, err := db.Exec(fmt.Sprintf(
 			`ALTER TABLE _async_migrations ADD COLUMN %s %s`, c.name, c.typ))
 		if err != nil && !isDuplicateColumnErr(err) {

cmd/server/async_migrations_test.go

@@ -18,6 +18,7 @@ func openAsyncTestDB(t *testing.T) *sql.DB {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { db.Close() })
+	// PREFLIGHT: async=true reason="test fixture CREATE TABLE on a fresh in-memory SQLite DB — not a real schema migration; runs in test setup only."
 	_, err = db.Exec(`
 		CREATE TABLE _async_migrations (
 			name           TEXT PRIMARY KEY,