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

scripts/pb-init-v3.sh

@@ -272,5 +272,7 @@ add_field "app_users"      "verification_token"     "text"
 add_field "app_users"      "verification_token_exp" "text"
 add_field "app_users"      "oauth_provider"         "text"
 add_field "app_users"      "oauth_id"               "text"
+add_field "app_users"      "polar_customer_id"      "text"
+add_field "app_users"      "polar_subscription_id"  "text"
 
 log "done"