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/cache/conversation_cache.go

@@ -692,36 +692,6 @@ func (c *ConversationCache) ComputeAllUnreadCount(ctx context.Context, userID st
 	return total, nil
 }
 
-// IncrementUnread 递增用户在某会话的未读数（已废弃：改用 seq 算术计算）
-// Deprecated: Unread count is now computed via arithmetic (maxSeq - readSeq).
-func (c *ConversationCache) IncrementUnread(ctx context.Context, userID, convID string) error {
-	return nil
-}
-
-// ClearUnread 清零用户在某会话的未读数（已废弃：改用 seq 算术计算）
-// Deprecated: No longer needed. Unread is computed from seq arithmetic.
-func (c *ConversationCache) ClearUnread(ctx context.Context, userID, convID string) error {
-	return nil
-}
-
-// GetUnreadCountFromHash 从 Redis Hash 获取单个会话未读数（已废弃）
-// Deprecated: Use ComputeUnreadCount instead.
-func (c *ConversationCache) GetUnreadCountFromHash(ctx context.Context, userID, convID string) (int64, error) {
-	return 0, ErrKeyNotFound
-}
-
-// GetAllUnreadCounts 从 Redis Hash 获取用户所有会话的未读数（已废弃）
-// Deprecated: Use ComputeAllUnreadCount instead.
-func (c *ConversationCache) GetAllUnreadCounts(ctx context.Context, userID string) (map[string]int64, error) {
-	return nil, ErrKeyNotFound
-}
-
-// GetTotalUnread 从 Redis 获取用户总未读数（已废弃）
-// Deprecated: Use ComputeAllUnreadCount instead.
-func (c *ConversationCache) GetTotalUnread(ctx context.Context, userID string) (int64, error) {
-	return 0, ErrKeyNotFound
-}
-
 // GetNextSeq 获取会话的下一个 seq 值（原子递增）
 // 使用 Redis INCR 实现，首次调用时从 DB 初始化并补齐差值
 func (c *ConversationCache) GetNextSeq(ctx context.Context, convID string) (int64, error) {