feat(channels): add LINE Official Account channel support

Add LINE Messaging API as the 9th messaging channel using HTTP Webhook.
Supports text/image/audio messages, group chat @mention detection,
reply with quote, and loading animation.

- No external SDK required (standard library only)
- HMAC-SHA256 webhook signature verification
- Reply Token (free) with Push API fallback
- Group chat: respond only when @mentioned
- Quote original message in replies using quoteToken

Closes #146

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/config.example.json

@@ -51,6 +51,15 @@
       "bot_token": "xoxb-YOUR-BOT-TOKEN",
       "app_token": "xapp-YOUR-APP-TOKEN",
       "allow_from": []
+    },
+    "line": {
+      "enabled": false,
+      "channel_secret": "YOUR_LINE_CHANNEL_SECRET",
+      "channel_access_token": "YOUR_LINE_CHANNEL_ACCESS_TOKEN",
+      "webhook_host": "0.0.0.0",
+      "webhook_port": 18791,
+      "webhook_path": "/webhook/line",
+      "allow_from": []
     }
   },
   "providers": {