refactor(cache): remove deprecated unread count methods and keys

Clean up the codebase by removing unused and deprecated unread count
functionality in `ConversationCache` and associated Redis key
generators. This follows the transition to sequence-based unread
calculation.

- Remove `IncrementUnread`, `ClearUnread`, `GetUnreadCountFromHash`,
  `GetAllUnreadCounts`, and `GetTotalUnread` from `ConversationCache`.
- Remove `PrefixUnreadHash` and `PrefixUnreadTotal` constants and
  `UnreadHashKey`/`UnreadTotalKey` helper functions.
- Remove unused `isValidContentType` helper in `MessageHandler`.

internal/handler/message_handler.go

@@ -653,16 +653,6 @@ func (h *MessageHandler) DeleteMessage(c *gin.Context) {
 	response.SuccessWithMessage(c, "message deleted", nil)
 }
 
-// 辅助函数：验证内容类型
-func isValidContentType(contentType model.ContentType) bool {
-	switch contentType {
-	case model.ContentTypeText, model.ContentTypeImage, model.ContentTypeVideo, model.ContentTypeAudio, model.ContentTypeFile:
-		return true
-	default:
-		return false
-	}
-}
-
 // 辅助函数：获取会话参与者信息
 func (h *MessageHandler) getConversationParticipants(ctx context.Context, conversationID string, currentUserID string) ([]*model.User, error) {
 	// 从repository获取参与者列表