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/cache/redis_cache.go

@@ -222,6 +222,12 @@ func (c *RedisCache) HDel(ctx context.Context, key string, fields ...string) err
 	return c.client.HDel(ctx, key, fields...)
 }
 
+// HIncrBy 原子递增 Hash 字段的值
+func (c *RedisCache) HIncrBy(ctx context.Context, key string, field string, incr int64) (int64, error) {
+	key = normalizeKey(key)
+	return c.client.HIncrBy(ctx, key, field, incr)
+}
+
 // ==================== RedisCache Sorted Set 操作 ====================
 
 // ZAdd 添加 Sorted Set 成员
@@ -281,6 +287,13 @@ func (c *RedisCache) Incr(ctx context.Context, key string) (int64, error) {
 	return c.client.Incr(ctx, key)
 }
 
+// IncrBySeq 原子递增指定值（用于 seq 对齐）
+func (c *RedisCache) IncrBySeq(ctx context.Context, key string, delta int64) (int64, error) {
+	key = normalizeKey(key)
+	rdb := c.client.GetClient()
+	return rdb.IncrBy(ctx, key, delta).Result()
+}
+
 // Expire 设置过期时间
 func (c *RedisCache) Expire(ctx context.Context, key string, ttl time.Duration) error {
 	key = normalizeKey(key)