feat(messages): implement integrated chat with voice/calls and tidy root go module

Add Discord-like messaging APIs, websocket realtime, smart suggestions, password vault flows, semantic indexing integration, and new /app/messages UI.

Add typing indicators, advanced message search filters, voice notes, browser-local optional transcription, and WebRTC call signaling (offer/answer/ice/hangup).

Clean root go.mod via go mod tidy and remove stale root go.sum.

backend/models/models.go

@@ -113,5 +113,15 @@ func AutoMigrate() {
 		&YouTubeChannelCache{},
 		// Video bookmark models
 		&VideoBookmark{},
+		// Messaging models
+		&Conversation{},
+		&ConversationMember{},
+		&Message{},
+		&MessageAttachment{},
+		&MessageReference{},
+		&MessageSuggestion{},
+		&MessageReaction{},
+		&PasswordVaultItem{},
+		&PasswordVaultShare{},
 	)
 }