feat(cache): implement database fallback for conversation sequence management

Improve the reliability of conversation sequence generation by implementing a fallback mechanism to the database when Redis is unavailable or when keys are missing.

- Add `GetNextSeq` to `MessageRepository` and implement it in `MessageRepositoryAdapter`.
- Update `ConversationCache` to fallback to DB in `GetConvMaxSeq`, `GetConvMaxSeqBatch`, and `GetNextSeq`.
- Ensure Redis TTL is refreshed after every `INCR` operation to prevent sequence counter expiration.
- Refactor service layers to handle sequence generation errors more explicitly.
- Add `IsEnabled` helper to `LayeredCache` for cleaner availability checks.

internal/cache/repository_adapter.go

@@ -74,3 +74,8 @@ func (a *MessageRepositoryAdapter) CreateMessage(msg *model.Message) error {
 func (a *MessageRepositoryAdapter) UpdateConversationLastSeq(convID string, seq int64) error {
 	return a.repo.UpdateConversationLastSeq(convID, seq)
 }
+
+// GetNextSeq 实现 MessageRepository 接口
+func (a *MessageRepositoryAdapter) GetNextSeq(convID string) (int64, error) {
+	return a.repo.GetNextSeq(convID)
+}