fix: preserve registration metadata through Ticket Tailor widget

[Mar-IA-no]

Outcome

Fixes the production checkout failure reported after a successful canonical registration.

The landing previously navigated directly to checkout.widget_url. Ticket Tailor supports p[meta_*] only when that URL is consumed by an event-specific widget, not as a direct checkout navigation.

Change

• mounts the official Ticket Tailor inline widget after registration.response.v1;
• passes the already validated widget URL as data-url;
• preserves meta_registration_context in the fragment;
• removes direct location.assign checkout navigation;
• fails closed if the widget script cannot load;
• does not touch Bot, PostgreSQL, Beacon, events, prices or legal versions.

Evidence

• 23/23 tests pass;
• static production build passes;
• diff-check passes;
• official widget script responds HTTP 200;
• tests cover exact metadata preservation, invalid origins and script-load failure.

Official contract:
https://help.tickettailor.com/en/articles/9859154-how-to-send-customer-data-to-your-widget

Independent hotfix over main; draft PRs #22 and #16 are untouched and can rebase after merge.

Closes #23.