feat(channel): improve cache strategy and add restriction logging

- Change channel cache TTL from 60s to 10min (reduce unnecessary DB queries)
- Actively rebuild cache after CRUD instead of lazy invalidation
- Add slog.Warn logging for channel pricing restriction blocks (4 places)

backend/internal/service/openai_gateway_service.go

@@ -1202,6 +1202,9 @@ func (s *OpenAIGatewayService) SelectAccountForModelWithExclusions(ctx context.C
 
 func (s *OpenAIGatewayService) selectAccountForModelWithExclusions(ctx context.Context, groupID *int64, sessionHash string, requestedModel string, excludedIDs map[int64]struct{}, stickyAccountID int64) (*Account, error) {
 	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
+		slog.Warn("channel pricing restriction blocked request",
+			"group_id", derefGroupID(groupID),
+			"model", requestedModel)
 		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
 	}
 
@@ -1379,6 +1382,9 @@ func (s *OpenAIGatewayService) isBetterAccount(candidate, current *Account) bool
 // SelectAccountWithLoadAwareness selects an account with load-awareness and wait plan.
 func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Context, groupID *int64, sessionHash string, requestedModel string, excludedIDs map[int64]struct{}) (*AccountSelectionResult, error) {
 	if s.checkChannelPricingRestriction(ctx, groupID, requestedModel) {
+		slog.Warn("channel pricing restriction blocked request",
+			"group_id", derefGroupID(groupID),
+			"model", requestedModel)
 		return nil, fmt.Errorf("%w supporting model: %s (channel pricing restriction)", ErrNoAvailableAccounts, requestedModel)
 	}