Event cancellation, payment, and analytics enhancements

A cluster of smaller but real improvements across services.

Event cancellation: a JWT-authenticated cancel endpoint in the bookings API, with a guard ensuring an event that had already ended or started couldn't be cancelled after the fact, plus cancellation notification templates for guides, participants, and initiators, each its own C#/.NET Core service tested with XUnit and Moq, a genuinely separate stack from the TypeScript the rest of this cluster runs on.

Payments: I fixed VAT rate and VAT ID mapping across the pricing and receipt paths, correctness work that matters because getting VAT wrong on an invoice is a compliance problem, not just a display bug.

Analytics: Athena-based querying for receipt and conversion metrics, tightening correctness in what counted as a real, completed transaction. On the marketing side, I wired Google Tag Manager to GA4 and Meta Pixel for front-end tracking, added a Google Ads integration behind a feature flag, and built receipt-conversion tracking so marketing spend could actually be measured against real completed bookings, not just page views.