feat(tracking): push Google Ads conversion data to dataLayer on purchase

[paulocastellano]

Summary

• Wires Stripe Checkout Session data (value, currency, transaction_id) into the purchase dataLayer event so GTM can fire Google Ads Conversion Tracking (AW-18156274196/Dl3oCNKAvKscEJSEy9FD) with accurate per-plan revenue and deduped transaction IDs.
• BillingController::checkout now appends {CHECKOUT_SESSION_ID} to the Stripe success_url.
• BillingController::processing lazily retrieves the Checkout Session (closure prop — polling partial reloads don't re-hit the Stripe API) and exposes a conversion prop with value/currency/transaction_id. Uses data_get with dot notation + type guards so Stripe shape changes degrade to null instead of crashing.
• Processing.vue forwards conversion to trackPurchase.
• useTracking.trackPurchase pushes conversion_value, conversion_currency, conversion_transaction_id to dataLayer + PostHog.

GTM setup (manual, after merge)

1. Create 3 Data Layer Variables: conversion_value, conversion_currency, conversion_transaction_id.
2. Create a Google Ads Conversion Tracking tag:
   • Conversion ID: 18156274196 (digits only, no AW- prefix)
   • Conversion Label: Dl3oCNKAvKscEJSEy9FD
   • Conversion Value: {{conversion_value}}
   • Transaction ID: {{conversion_transaction_id}}
   • Currency Code: {{conversion_currency}}
3. Trigger: Custom Event = purchase.

Test plan

• php artisan test --compact --filter=BillingControllerTest — 20 passed
• Manual checkout in Stripe test mode → confirm purchase event in GTM Preview with populated conversion_* fields
• Verify Google Ads Conversion Tracking tag fires once per checkout (dedup via transaction_id)
• Confirm polling partial reloads on /billing/processing don't trigger duplicate Stripe API calls