FRE-4510: Add voiceprint feature flag support

- Create voiceprint.feature-flags.ts to re-export checkFlag
- Update voiceprint.config.ts to use checkFlag for all flags
- Update voiceprint.service.ts to import checkFlag
- Ensure voiceprint services respect feature flags

apps/api/src/services/voiceprint/voiceprint.feature-flags.ts

@@ -0,0 +1,7 @@
+/**
+ * VoicePrint Feature Flags
+ * Re-exports the checkFlag function from the centralized feature flag system
+ */
+
+// Re-export the checkFlag function from the spamshield feature flags module
+export { checkFlag } from '../spamshield/feature-flags';