feat: add auth.disableSignUp config option

- Added disableSignUp to authConfigSchema in config-schema.ts
- Added authDisableSignUp to Config interface
- Added parsing from PAPERCLIP_AUTH_DISABLE_SIGN_UP env or config file
- Passed to better-auth emailAndPassword.disableSignUp

When true, blocks new user registrations on public instances.
Defaults to false (backward compatible).

Fixes #241

server/src/auth/better-auth.ts

@@ -86,6 +86,7 @@ export function createBetterAuthInstance(db: Db, config: Config, trustedOrigins?
     emailAndPassword: {
       enabled: true,
       requireEmailVerification: false,
+      disableSignUp: config.authDisableSignUp ?? false,
     },
   };