Add comprehensive multi-API integration support

This commit adds extensive API integration capabilities for deployment,
payments, communications, and monitoring to BlackRoad OS.

New API Integrations:
- Railway API: Cloud deployment management (GraphQL)
- Vercel API: Serverless deployment platform (REST)
- Stripe API: Payment processing and billing
- Twilio API: SMS, Voice, and WhatsApp messaging
- Slack API: Team collaboration and notifications
- Discord API: Community messaging and notifications
- Sentry API: Error tracking and application monitoring

Core Features:
- Centralized API client manager with health checking
- Comprehensive health monitoring endpoint (/api/health/*)
- Automatic retry logic and rate limit handling
- Unified status monitoring for all integrations

Infrastructure:
- Railway deployment configuration (railway.json, railway.toml)
- Enhanced GitHub Actions workflows:
  * backend-tests.yml: Comprehensive test suite with PostgreSQL/Redis
  * railway-deploy.yml: Automated Railway deployment with notifications
- Docker build validation in CI/CD pipeline

Testing:
- Comprehensive test suite for all API integrations
- API connectivity verification in CI/CD
- Mock-friendly architecture for testing without credentials

Configuration:
- Updated .env.example with all new API keys
- Added stripe and sentry-sdk to requirements.txt
- Registered all new routers in main.py
- Updated API info endpoint with new integrations

Documentation:
- API_INTEGRATIONS.md: Complete setup and usage guide
- Interactive API docs at /api/docs with all endpoints
- Health check endpoints for monitoring

All APIs are optional and gracefully handle missing credentials.
The system provides clear status messages for configuration requirements.

backend/requirements.txt

@@ -56,3 +56,9 @@ prometheus-client==0.19.0
 
 # CORS
 python-cors==1.0.0
+
+# New API Integrations (Railway, Vercel, Stripe, Twilio, Slack, Discord, Sentry)
+# Note: Most integrations use httpx (already included above)
+# Additional packages for specific integrations:
+stripe==7.8.0
+sentry-sdk==1.39.1