ActivityPub background actor validation, update and cleanup process (#4765)

* feat(ap): add validation+update+cleanup AP actor job

* fix(test): use time.Time instead of sql.NullTime

* feat(test): add integration test for AP follower cleanup/validation job

persistence/tables/migrations.go

@@ -35,6 +35,8 @@ func MigrateDatabaseSchema(db *sql.DB, from, to int) error {
 			migrateToSchema7(db)
 		case 7:
 			migrateToSchema8(db)
+		case 8:
+			migrateToSchema9(db)
 		default:
 			log.Fatalln("missing database migration step")
 		}
@@ -48,6 +50,21 @@ func MigrateDatabaseSchema(db *sql.DB, from, to int) error {
 	return nil
 }
 
+func migrateToSchema9(db *sql.DB) {
+	// Add columns for tracking follower validation status.
+	// last_validated_at: when the follower was last successfully validated
+	// first_validation_failure_at: when consecutive validation failures started (for removal threshold)
+	alterStmts := []string{
+		"ALTER TABLE ap_followers ADD COLUMN last_validated_at TIMESTAMP",
+		"ALTER TABLE ap_followers ADD COLUMN first_validation_failure_at TIMESTAMP",
+	}
+	for _, stmt := range alterStmts {
+		if _, err := db.Exec(stmt); err != nil {
+			log.Warnln("Migration statement may have already been applied:", err)
+		}
+	}
+}
+
 func migrateToSchema8(db *sql.DB) {
 	// Add shared_inbox column to ap_followers for ActivityPub shared inbox support.
 	// This allows delivering messages to a server's shared inbox instead of each