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

docker-compose.yml

@@ -92,8 +92,9 @@ services:
       REDIS_URL: redis://redis:6379
       API_GATEWAY_URL: http://api-gateway:8000
       WHATSAPP_CORE_URL: http://whatsapp-core:3001
-      OPENAI_API_KEY: ${OPENAI_API_KEY:-}
-      OPENAI_MODEL: ${OPENAI_MODEL:-gpt-4o-mini}
+      DEEPSEEK_API_KEY: ${DEEPSEEK_API_KEY:-}
+      DEEPSEEK_MODEL: ${DEEPSEEK_MODEL:-deepseek-chat}
+      DEEPSEEK_BASE_URL: ${DEEPSEEK_BASE_URL:-https://api.deepseek.com}
     depends_on:
       postgres:
         condition: service_healthy