Add feature flags, seed data utilities, and Angular frontend scaffold

Major enhancements:
- Feature flag system for cloud vs air-gapped deployment modes
- Automatic storage backend selection based on deployment mode
- Comprehensive seed data generation utilities
- Support for generating CSV, JSON, binary, and PCAP test files
- Quick seed script for easy data generation
- Angular 19 frontend complete setup documentation
- Material Design UI component examples and configuration

Fixes:
- Resolve SQLAlchemy metadata column name conflict
- Rename metadata to custom_metadata throughout codebase
- Fix API health check issues

Documentation:
- FEATURES.md - Complete feature overview
- FRONTEND_SETUP.md - Angular 19 setup guide with examples
- SUMMARY.md - Implementation summary

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -1,7 +1,12 @@
+# Deployment Mode (Feature Flag)
+# Options: "cloud" or "air-gapped"
+# cloud = automatically uses S3, air-gapped = automatically uses MinIO
+DEPLOYMENT_MODE=air-gapped
+
 # Database Configuration
 DATABASE_URL=postgresql://user:password@localhost:5432/datalake
 
-# Storage Backend Configuration
+# Storage Backend Configuration (optional - auto-configured based on DEPLOYMENT_MODE)
 # Options: "s3" or "minio"
 STORAGE_BACKEND=minio