Wire `value`, `currency`, and `transaction_id` from Stripe Checkout Session
into the `purchase` dataLayer event so GTM can fire Google Ads Conversion
Tracking with accurate per-plan revenue and deduped transaction IDs.
- `BillingController::checkout` adds `{CHECKOUT_SESSION_ID}` to success_url
- `BillingController::processing` retrieves session lazily (closure prop,
so polling partial reloads don't re-hit Stripe) and exposes
`conversion` with `value`/`currency`/`transaction_id`
- `Processing.vue` forwards `conversion` to `trackPurchase`
- `useTracking.trackPurchase` pushes `conversion_value`,
`conversion_currency`, `conversion_transaction_id` to dataLayer +
PostHog
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