feat: improve model rate limit tracking

apps/api/internal/store/runtime_policies.go

@@ -134,6 +134,21 @@ WHERE id = $1::uuid`, platformID, cooldownSeconds)
 	return err
 }
 
+func (s *Store) CooldownCandidatePlatformModel(ctx context.Context, platformModelID string, cooldownSeconds int) error {
+	if strings.TrimSpace(platformModelID) == "" {
+		return nil
+	}
+	if cooldownSeconds <= 0 {
+		cooldownSeconds = 300
+	}
+	_, err := s.pool.Exec(ctx, `
+UPDATE platform_models
+SET cooldown_until = now() + ($2::int * interval '1 second'),
+    updated_at = now()
+WHERE id = $1::uuid`, platformModelID, cooldownSeconds)
+	return err
+}
+
 func (s *Store) DemoteCandidatePlatformPriority(ctx context.Context, platformID string, demoteStep int) error {
 	if strings.TrimSpace(platformID) == "" {
 		return nil