Add README

frontend/src/api/README.md

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+frontend/src/api/
+├── client/                          # HTTP client configuration
+│   ├── index.ts                     # Main API client
+│   ├── config.ts                    # Client configuration
+│   ├── interceptors.ts              # Request/response interceptors
+│   └── types.ts                     # Client-specific types
+├── services/                        # Service-specific API calls
+│   ├── index.ts                     # Export all services
+│   ├── auth.service.ts              # Authentication operations
+│   ├── tenant.service.ts            # Tenant management
+│   ├── data.service.ts              # Data operations
+│   ├── training.service.ts          # ML training operations
+│   ├── forecasting.service.ts       # Forecasting operations
+│   └── notification.service.ts      # Notification operations
+├── types/                           # TypeScript definitions
+│   ├── index.ts                     # Re-export all types
+│   ├── common.ts                    # Common API types
+│   ├── auth.ts                      # Authentication types
+│   ├── tenant.ts                    # Tenant types
+│   ├── data.ts                      # Data types
+│   ├── training.ts                  # Training types
+│   ├── forecasting.ts               # Forecasting types
+│   └── notification.ts              # Notification types
+├── hooks/                           # React hooks for API calls
+│   ├── index.ts                     # Export all hooks
+│   ├── useAuth.ts                   # Authentication hooks
+│   ├── useTenant.ts                 # Tenant hooks
+│   ├── useData.ts                   # Data hooks
+│   ├── useTraining.ts               # Training hooks
+│   ├── useForecast.ts               # Forecasting hooks
+│   └── useNotification.ts           # Notification hooks
+├── utils/                           # API utilities
+│   ├── index.ts                     # Export utilities
+│   ├── response.ts                  # Response handling
+│   ├── error.ts                     # Error handling
+│   ├── validation.ts                # Request validation
+│   └── transform.ts                 # Data transformation
+├── websocket/                       # WebSocket management
+│   ├── index.ts                     # WebSocket exports
+│   ├── manager.ts                   # WebSocket manager
+│   ├── types.ts                     # WebSocket types
+│   └── hooks.ts                     # WebSocket hooks
+└── index.ts                         # Main API exports
+```
+
+## 🎯 Key Improvements
+
+### 1. **Modern Architecture Patterns**
+- **Service Layer Pattern**: Clean separation of concerns
+- **Repository Pattern**: Consistent data access layer
+- **Factory Pattern**: Flexible service instantiation
+- **Observer Pattern**: Event-driven updates
+
+### 2. **Type Safety**
+- **Strict TypeScript**: Full type coverage
+- **Schema Validation**: Runtime type checking
+- **Generic Types**: Reusable type definitions
+- **Union Types**: Precise API responses
+
+### 3. **Error Handling**
+- **Centralized Error Management**: Consistent error handling
+- **Error Recovery**: Automatic retry mechanisms
+- **User-Friendly Messages**: Localized error messages
+- **Error Boundaries**: Component-level error isolation
+
+### 4. **Performance Optimization**
+- **Request Caching**: Intelligent cache management
+- **Request Deduplication**: Prevent duplicate calls
+- **Optimistic Updates**: Immediate UI feedback
+- **Background Sync**: Offline-first approach
+
+### 5. **Developer Experience**
+- **Auto-completion**: Full IntelliSense support
+- **Type-safe Hooks**: React hooks with types
+- **Error Prevention**: Compile-time error detection
+- **Documentation**: Comprehensive JSDoc comments
+
+## 🚀 Implementation Benefits
+
+1. **Maintainability**: Modular structure for easy updates
+2. **Scalability**: Easy to add new services and endpoints
+3. **Testability**: Isolated services for unit testing
+4. **Reusability**: Shared utilities and types
+5. **Type Safety**: Prevent runtime errors
+6. **Developer Productivity**: IntelliSense and auto-completion