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>

app/main.py

@@ -40,6 +40,7 @@ async def startup_event():
     """Initialize database on startup"""
     logger.info("Initializing database...")
     init_db()
+    logger.info(f"Deployment mode: {settings.deployment_mode}")
     logger.info(f"Using storage backend: {settings.storage_backend}")
     logger.info("Application started successfully")
 
@@ -51,6 +52,7 @@ async def root():
         "message": "Test Artifact Data Lake API",
         "version": "1.0.0",
         "docs": "/docs",
+        "deployment_mode": settings.deployment_mode,
         "storage_backend": settings.storage_backend
     }