fix(api): await PostHog flush at conversion chokepoints (webhook events were lost)

payment_success / payment_failed / subscription_activated are captured inside
the Stripe webhook handler and activateSubscription — short requests that return
immediately. posthog-node's fire-and-forget flush was abandoned before the send
completed, so these events were written to the DB but never reached PostHog
(DB had 4 completed Stripe payments in 30d; PostHog had 1 payment_success and 0
payment_failed). payment_initiated, fired in a normal user request, landed fine —
which is what isolated the cause to the webhook/short-request context.

Add PostHogService.flush() and await it at the end of handleWebhook and after the
subscription_activated capture in activateSubscription (the shared Stripe+EFT
chokepoint). Restores server-side paid-conversion visibility so trial→paid ROI is
measurable in PostHog instead of only the DB.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/subscriptions/subscriptions.service.ts

@@ -192,6 +192,11 @@ export class SubscriptionsService {
       referral_credit_days: creditDays,
     });
 
+    // Await delivery: activateSubscription is the shared chokepoint for Stripe
+    // (webhook) AND EFT/manual activation, both of which run in short requests
+    // whose fire-and-forget flush was dropping this revenue event.
+    await this.posthog.flush();
+
     return updated;
   }