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

packages/shared/src/config-schema.ts

@@ -55,6 +55,7 @@ export const serverConfigSchema = z.object({
 export const authConfigSchema = z.object({
   baseUrlMode: z.enum(AUTH_BASE_URL_MODES).default("auto"),
   publicBaseUrl: z.string().url().optional(),
+  disableSignUp: z.boolean().default(false),
 });
 
 export const storageLocalDiskConfigSchema = z.object({
@@ -103,6 +104,7 @@ export const paperclipConfigSchema = z
     server: serverConfigSchema,
     auth: authConfigSchema.default({
       baseUrlMode: "auto",
+      disableSignUp: false,
     }),
     storage: storageConfigSchema.default({
       provider: "local_disk",