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/gateway_service.go

@@ -1199,6 +1199,9 @@ func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context
 	// Claude Code 限制可能已将 groupID 解析为 fallback group，
 	// 渠道限制预检查必须使用解析后的分组。
 	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)
 	}
 
@@ -1241,6 +1244,9 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 	// Claude Code 限制可能已将 groupID 解析为 fallback group，
 	// 渠道限制预检查必须使用解析后的分组。
 	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)
 	}