feat(core): optimize performance and reliability through batching and redis-backed unread counts

This commit introduces several significant architectural improvements to enhance system performance, scalability, and reliability:

- **Performance Optimization (N+1 Problem Resolution)**: Implemented batching mechanisms across `MessageHandler`, `ChatService`, `GroupService`, `MessageService`, and `UserService`. This replaces multiple individual database/cache queries with single batch operations for fetching unread counts, last messages, participants, and user information.
- **Enhanced Unread Count Management**: Migrated unread count tracking to a Redis Hash-based approach (`unread#️⃣{userID}`). This allows for atomic increments/decrements and efficient retrieval of both individual conversation unread counts and total unread counts for a user.
- **Improved Message Sequencing**: Integrated Redis-based sequence (`seq`) pre-allocation for messages to ensure strict ordering and reduce database contention during high-concurrency message creation.
- **Push Service Reliability**: Refactored the push notification system to include persistent `PushRecord` tracking. Added a recovery mechanism (`recoverPendingPushes`) to reload pending notifications from the database upon service startup, ensuring better delivery guarantees.
- **WebSocket Reliability**: Updated the WebSocket hub to move away from in-memory history replay in favor of a client-driven synchronization model (`sync_required` event and `ack` handling), reducing memory overhead and improving connection stability.
- **Cache Layer Enhancements**: Added `HIncrBy` and `IncrBySeq` to the `Cache` interface and its implementations (`RedisCache`, `LayeredCache`) to support the new unread and sequence management logic.

internal/repository/user_repo.go

@@ -1,6 +1,7 @@
 package repository
 
 import (
+	"context"
 	"strings"
 	"time"
 	"with_you/internal/model"
@@ -13,6 +14,7 @@ import (
 type UserRepository interface {
 	Create(user *model.User) error
 	GetByID(id string) (*model.User, error)
+	GetUsersByIDs(ctx context.Context, ids []string) (map[string]*model.User, error)
 	GetByUsername(username string) (*model.User, error)
 	GetByEmail(email string) (*model.User, error)
 	GetByPhone(phone string) (*model.User, error)
@@ -607,3 +609,22 @@ func (r *userRepository) GetPendingDeletionUsers(beforeTime time.Time) ([]*model
 func (r *userRepository) HardDeleteUser(userID string) error {
 	return r.db.Unscoped().Delete(&model.User{}, "id = ?", userID).Error
 }
+
+// GetUsersByIDs 批量获取用户信息
+func (r *userRepository) GetUsersByIDs(ctx context.Context, ids []string) (map[string]*model.User, error) {
+	result := make(map[string]*model.User, len(ids))
+	if len(ids) == 0 {
+		return result, nil
+	}
+
+	var users []*model.User
+	err := r.db.WithContext(ctx).Where("id IN ?", ids).Find(&users).Error
+	if err != nil {
+		return nil, err
+	}
+
+	for _, u := range users {
+		result[u.ID] = u
+	}
+	return result, nil
+}