feat(conversations): add notification mute functionality for conversations

- Rename Muted field to NotificationMuted across model, DTOs and converters
- Add UpdateNotificationMuted repository method with upsert support
- Add SetConversationNotificationMuted service method
- Add PUT /:id/notification_muted route for setting mute status
- Improve S3 storage resilience by skipping bucket check on access denied

internal/model/conversation.go

@@ -56,7 +56,7 @@ type ConversationParticipant struct {
 	ConversationID string     `gorm:"not null;size:20;uniqueIndex:idx_conversation_user,priority:1;index:idx_cp_conversation_user,priority:1" json:"conversation_id"`                                                                    // 雪花算法ID（string类型）
 	UserID         string     `gorm:"column:user_id;type:varchar(50);not null;uniqueIndex:idx_conversation_user,priority:2;index:idx_cp_conversation_user,priority:2;index:idx_cp_user_hidden_pinned_updated,priority:1" json:"user_id"` // UUID格式，与JWT中user_id保持一致
 	LastReadSeq    int64      `gorm:"default:0" json:"last_read_seq"`                                                                                                                                                                    // 已读到的seq位置
-	Muted          bool       `gorm:"default:false" json:"muted"`                                                                                                                                                                        // 是否免打扰
+	NotificationMuted bool       `gorm:"column:muted;default:false" json:"notification_muted"`                                                                                                                                                // 是否免打扰
 	IsPinned       bool       `gorm:"default:false;index:idx_cp_user_hidden_pinned_updated,priority:3" json:"is_pinned"`                                                                                                                 // 是否置顶会话（用户维度）
 	HiddenAt       *time.Time `gorm:"index:idx_cp_user_hidden_pinned_updated,priority:2" json:"hidden_at,omitempty"`                                                                                                                     // 仅自己删除会话时使用，收到新消息后自动恢复
 	CreatedAt      time.Time  `json:"created_at" gorm:"autoCreateTime"`