Add shared inbox support for ActivityPub delivery (#4755)

* feat(ap): add support for shared inboxes to reduce outbound load

* feat(db): refactor ap followers db into followers repository

* fix(ap): use the updated activity library to pull out the shared inbox endpoint

* chore(deps): point at updated build of owncast/activity

* fix(ap): typeless endpoints

* feat(test): update ActivityPub test to support shared inboxes

* chore(test): remove unused variable

* fix: feedback from review. Guard against SSRF/non-HTTPS/local and handle transaction errors

persistence/tables/migrations.go

@@ -33,6 +33,8 @@ func MigrateDatabaseSchema(db *sql.DB, from, to int) error {
 			migrateToSchema6(db)
 		case 6:
 			migrateToSchema7(db)
+		case 7:
+			migrateToSchema8(db)
 		default:
 			log.Fatalln("missing database migration step")
 		}
@@ -46,6 +48,23 @@ func MigrateDatabaseSchema(db *sql.DB, from, to int) error {
 	return nil
 }
 
+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
+	// user's individual inbox, reducing the number of outbound requests.
+	stmt, err := db.Prepare("ALTER TABLE ap_followers ADD COLUMN shared_inbox TEXT")
+	if err != nil {
+		log.Errorln("Error running migration. This may be because you have already been running a dev version.", err)
+		return
+	}
+	defer stmt.Close()
+
+	_, err = stmt.Exec()
+	if err != nil {
+		log.Warnln(err)
+	}
+}
+
 func migrateToSchema7(db *sql.DB) {
 	log.Println("Migrating users. This may take time if you have lots of users...")