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/fr.json

@@ -329,6 +329,18 @@
   "profile_password_changed_ok": "Mot de passe modifié avec succès.",
   "profile_playback_speed": "Vitesse de lecture — {speed}x",
 
+  "profile_subscription_heading": "Abonnement",
+  "profile_plan_pro": "Pro",
+  "profile_plan_free": "Gratuit",
+  "profile_pro_active": "Votre abonnement Pro est actif.",
+  "profile_pro_perks": "Audio illimité, toutes les langues de traduction et la sélection de voix sont activées.",
+  "profile_manage_subscription": "Gérer l'abonnement",
+  "profile_upgrade_heading": "Passer au Pro",
+  "profile_upgrade_desc": "Débloquez l'audio illimité, les traductions en 4 langues et la sélection de voix.",
+  "profile_upgrade_monthly": "Mensuel — 6 $ / mois",
+  "profile_upgrade_annual": "Annuel — 48 $ / an",
+  "profile_free_limits": "Plan gratuit : 3 chapitres audio par jour, lecture en anglais uniquement.",
+
   "user_currently_reading": "En cours de lecture",
   "user_library_count": "Bibliothèque ({n})",
   "user_joined": "Inscrit le {date}",