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/handler/qrcode_handler.go

@@ -53,7 +53,7 @@ func (h *QRCodeHandler) WSEvents(c *gin.Context) {
 		return
 	}
 
-	ch, cancel, replay := h.qrcodeService.GetWSHub().Subscribe(sessionID, 0)
+	ch, cancel := h.qrcodeService.GetWSHub().Subscribe(sessionID, 0)
 	defer cancel()
 
 	w := c.Writer
@@ -82,13 +82,6 @@ func (h *QRCodeHandler) WSEvents(c *gin.Context) {
 		return true
 	}
 
-	// 发送历史事件
-	for _, ev := range replay {
-		if !writeEvent(ev) {
-			return
-		}
-	}
-
 	// 心跳
 	heartbeat := time.NewTicker(25 * time.Second)
 	defer heartbeat.Stop()