refactor(ai): replace OpenAI with DeepSeek API

- Update config.py with DEEPSEEK_API_KEY, DEEPSEEK_MODEL, DEEPSEEK_BASE_URL
- Update ai.py to use DeepSeek endpoint (OpenAI-compatible)
- Update docker-compose.yml environment variables

services/flow-engine/app/config.py

@@ -6,9 +6,10 @@ class Settings(BaseSettings):
     DATABASE_URL: str = "postgresql://whatsapp_admin:password@localhost:5432/whatsapp_central"
     REDIS_URL: str = "redis://localhost:6379"
 
-    # OpenAI
-    OPENAI_API_KEY: str = ""
-    OPENAI_MODEL: str = "gpt-3.5-turbo"
+    # DeepSeek AI (OpenAI-compatible API)
+    DEEPSEEK_API_KEY: str = ""
+    DEEPSEEK_MODEL: str = "deepseek-chat"
+    DEEPSEEK_BASE_URL: str = "https://api.deepseek.com"
 
     API_GATEWAY_URL: str = "http://localhost:8000"
     WHATSAPP_CORE_URL: str = "http://localhost:3001"