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>

pkg/channels/manager.go

@@ -149,6 +149,19 @@ func (m *Manager) initChannels() error {
 		}
 	}
 
+	if m.config.Channels.LINE.Enabled && m.config.Channels.LINE.ChannelAccessToken != "" {
+		logger.DebugC("channels", "Attempting to initialize LINE channel")
+		line, err := NewLINEChannel(m.config.Channels.LINE, m.bus)
+		if err != nil {
+			logger.ErrorCF("channels", "Failed to initialize LINE channel", map[string]interface{}{
+				"error": err.Error(),
+			})
+		} else {
+			m.channels["line"] = line
+			logger.InfoC("channels", "LINE channel enabled successfully")
+		}
+	}
+
 	logger.InfoCF("channels", "Channel initialization completed", map[string]interface{}{
 		"enabled_channels": len(m.channels),
 	})