FRE-4510: Implement feature flag checks for spam classification

- Create centralized feature flag management system (feature-flags.ts)
- Add 15 feature flags across SpamShield, VoicePrint, and Platform categories
- Update spamshield.config.ts to use checkFlag() for all flags
- Add feature flag checks to all spamshield.service.ts methods:
  * NumberReputationService.checkReputation()
  * NumberReputationService.checkMultiSource()
  * SMSClassifierService.classify()
  * CallAnalysisService.analyzeCall()
  * SpamFeedbackService.recordFeedback()
- Update index.ts exports to include feature flag utilities
- Flags support runtime updates via FLAG_<KEY> environment variables

Flags implemented:
- SpamShield: enableNumberReputation, enableContentClassification,
  enableBehavioralAnalysis, enableCommunityIntelligence,
  enableRealTimeBlocking, enableMultipleSources, enableMLClassifier
- VoicePrint: enableMLService, enableFAISSIndex, enableBatchAnalysis,
  enableRealtimeAnalysis, enableMockModel
- Platform: enableAuditLogs, enableKPITracking

apps/api/src/services/voiceprint/index.ts

@@ -8,8 +8,12 @@ export {
   audioPreprocessingConfig,
   voicePrintFeatureFlags,
   voicePrintRateLimits,
+  checkFlag,
+  isFeatureEnabled,
 } from './voiceprint.config';
 
+
+
 // Services
 export {
   AudioPreprocessor,