feat: Implement comprehensive OpenAPI specification

Implements complete OpenAPI 3.0 specification for Cloud Safeguard API:
- Comprehensive API documentation with all endpoints
- Authentication methods documentation (X-User, Bearer, ApiKey)
- RBAC permissions and roles documentation
- Complete request/response schemas
- Error handling documentation
- CORS support documentation
- WebSocket endpoint documentation
- Usage examples and SDK integration guides

Files added:
- docs/api/openapi.yaml - Complete OpenAPI 3.0 specification
- docs/api/README.md - Comprehensive API documentation
- internal/web/openapi_test.go - OpenAPI endpoint tests

Files modified:
- internal/web/server.go - Added OpenAPI endpoints (/api/openapi.yaml, /api/openapi.json)
- internal/web/api.go - Implemented OpenAPI handlers with embedded fallback

Features:
- Public OpenAPI endpoints (no authentication required)
- YAML and JSON format support
- Embedded specification fallback for testing
- CORS headers for cross-origin access
- Comprehensive endpoint coverage
- Authentication and authorization documentation
- Usage examples and SDK integration guides

API Endpoints:
- GET /api/openapi.yaml - OpenAPI specification (YAML)
- GET /api/openapi.json - OpenAPI specification (JSON)

Phase 3 Progress: OpenAPI Specification ✅ Complete
Next: Security Agents, Documentation Updates, Testing