feat: add Layer 2 - WhatsApp Core logic, API Gateway models/auth, Frontend core

WhatsApp Core:
- SessionManager with Baileys integration for multi-account support
- Express server with REST API and Socket.IO for real-time events
- Session lifecycle management (create, disconnect, delete)
- Message sending with support for text, image, document, audio, video

API Gateway:
- Database models: User, WhatsAppAccount, Contact, Conversation, Message
- JWT authentication with access/refresh tokens
- Auth endpoints: login, refresh, register, me
- Pydantic schemas for request/response validation

Frontend:
- React 18 app structure with routing
- Zustand auth store with persistence
- API client with automatic token handling
- Base CSS and TypeScript declarations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/api-gateway/app/core/config.py

@@ -0,0 +1,30 @@
+from pydantic_settings import BaseSettings
+from functools import lru_cache
+
+
+class Settings(BaseSettings):
+    # Database
+    DATABASE_URL: str = "postgresql://whatsapp_admin:password@localhost:5432/whatsapp_central"
+
+    # Redis
+    REDIS_URL: str = "redis://localhost:6379"
+
+    # JWT
+    JWT_SECRET: str = "change-me-in-production"
+    JWT_ALGORITHM: str = "HS256"
+    JWT_ACCESS_TOKEN_EXPIRE_MINUTES: int = 60
+    JWT_REFRESH_TOKEN_EXPIRE_DAYS: int = 7
+
+    # WhatsApp Core
+    WHATSAPP_CORE_URL: str = "http://localhost:3001"
+
+    # CORS
+    CORS_ORIGINS: str = "http://localhost:5173,http://localhost:3000"
+
+    class Config:
+        env_file = ".env"
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings()