🐛 Billing emails: fall back to getBaseUrl() when config.app.url is unset #4128

What

• billing.email.js builds billingUrl and billingPortalUrl from config.app?.url. No Devkit default config sets that key, so the links render an empty href. Change it to config.app?.url || getBaseUrl(), so app.url still wins when a project sets it.
• In the same file, fix the JSDoc (lines 59-61) that describes a template "glob-merge" that does not exist.

Acceptance criteria

• With default config, the 3 billing builders (quota, credit, payment-failed) produce a non-empty link.
• A project that sets app.url gets exactly the same links as today.
• The JSDoc no longer claims a template override mechanism.

Tests

• billing.init.email-alerts.unit.tests.js: the link falls back to getBaseUrl() when app.url is unset, and app.url wins when it is set.

Independent. Merge first.
Refs: modules/billing/billing.email.js:59-61,70,108,138, lib/helpers/getBaseUrl.js, lib/services/express.js:106-112 (the other reader of app.url)