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/messages/id.json

@@ -329,6 +329,18 @@
   "profile_password_changed_ok": "Kata sandi berhasil diubah.",
   "profile_playback_speed": "Kecepatan pemutaran — {speed}x",
 
+  "profile_subscription_heading": "Langganan",
+  "profile_plan_pro": "Pro",
+  "profile_plan_free": "Gratis",
+  "profile_pro_active": "Langganan Pro kamu aktif.",
+  "profile_pro_perks": "Audio tanpa batas, semua bahasa terjemahan, dan pilihan suara tersedia.",
+  "profile_manage_subscription": "Kelola langganan",
+  "profile_upgrade_heading": "Tingkatkan ke Pro",
+  "profile_upgrade_desc": "Buka audio tanpa batas, terjemahan dalam 4 bahasa, dan pilihan suara.",
+  "profile_upgrade_monthly": "Bulanan — $6 / bln",
+  "profile_upgrade_annual": "Tahunan — $48 / thn",
+  "profile_free_limits": "Paket gratis: 3 bab audio per hari, hanya bahasa Inggris.",
+
   "user_currently_reading": "Sedang Dibaca",
   "user_library_count": "Perpustakaan ({n})",
   "user_joined": "Bergabung {date}",