fix: correct env var priority for authDisableSignUp

- Env var now properly overrides file config in both directions
- Follows established pattern for boolean config flags
- Removed redundant ?? false (field is typed boolean)
- PAPERCLIP_AUTH_DISABLE_SIGN_UP can now set to 'false' to
  override file config's 'true'

server/src/auth/better-auth.ts

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