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/en.json

@@ -329,6 +329,18 @@
   "profile_password_changed_ok": "Password changed successfully.",
   "profile_playback_speed": "Playback speed \u2014 {speed}x",
 
+  "profile_subscription_heading": "Subscription",
+  "profile_plan_pro": "Pro",
+  "profile_plan_free": "Free",
+  "profile_pro_active": "Your Pro subscription is active.",
+  "profile_pro_perks": "Unlimited audio, all translation languages, and voice selection are enabled.",
+  "profile_manage_subscription": "Manage subscription",
+  "profile_upgrade_heading": "Upgrade to Pro",
+  "profile_upgrade_desc": "Unlock unlimited audio, translations in 4 languages, and voice selection.",
+  "profile_upgrade_monthly": "Monthly \u2014 $6 / mo",
+  "profile_upgrade_annual": "Annual \u2014 $48 / yr",
+  "profile_free_limits": "Free plan: 3 audio chapters per day, English reading only.",
+
   "user_currently_reading": "Currently Reading",
   "user_library_count": "Library ({n})",
   "user_joined": "Joined {date}",