feat(billing): Polar.sh Pro subscription integration

- Webhook handler verifies HMAC-SHA256 sig and updates user role on
  subscription.created / subscription.updated / subscription.revoked
- Audio endpoint gated: free users limited to 3 chapters/day via Valkey
  counter; returns 402 {error:'pro_required'} when limit reached
- Translation proxy endpoint enforces 402 for non-pro users
- AudioPlayer.svelte surfaces 402 via onProRequired callback + upgrade banner
- Chapter page shows lock icon + upgrade prompts for gated translation langs
- Profile page: subscription section shows Pro badge + manage link (active)
  or monthly/annual checkout buttons (free); isPro resolved fresh from DB
- i18n: 13 new profile_subscription_* keys across all 5 locales

ui/src/app.d.ts

@@ -6,9 +6,11 @@ declare global {
 		interface Locals {
 			sessionId: string;
 			user: { id: string; username: string; role: string; authSessionId: string } | null;
+			isPro: boolean;
 		}
 		interface PageData {
 			user?: { id: string; username: string; role: string; authSessionId: string } | null;
+			isPro?: boolean;
 		}
 		// interface PageState {}
 		// interface Platform {}