更新环境配置，优化数据库和Redis连接设置

- 修改`.env.example`文件，更新数据库和Redis的默认主机地址为`192.168.9.127`
- 在`docker-compose.yml`中移除不必要的卷挂载配置
- 在`go.mod`中添加`github.com/joho/godotenv`依赖以支持环境变量加载
- 更新`README.md`以反映新的数据库和Redis连接信息
- 在`main.go`中加载环境变量以支持动态配置
- 新增缓存存在性检查功能，避免重复预热缓存
- 优化CSV读取逻辑，支持从S3获取数据

.env.example

@@ -6,23 +6,23 @@ APP_PORT=8080
 # Gin mode: debug | release | test
 GIN_MODE=debug
 
-# Database (development defaults apply when APP_ENV=development and vars are unset)
-# DATABASE_HOST=localhost
-# DATABASE_PORT=3306
-# DATABASE_NAME=audio
-# DATABASE_USER=root
-# DATABASE_PASSWORD=root123
+# Database (development reads from env, default host 192.168.9.127)
+DATABASE_HOST=192.168.9.127
+DATABASE_PORT=3306
+DATABASE_NAME=audio
+DATABASE_USER=root
+DATABASE_PASSWORD=root123
 
 # Meilisearch (development defaults apply when APP_ENV=development and vars are unset)
 # MEILISEARCH_HOST=http://ec2-18-184-205-87.eu-central-1.compute.amazonaws.com:7700
 # MEILISEARCH_API_KEY=your-api-key
 # MEILISEARCH_INDEX=models
 
-# Redis (development defaults apply when APP_ENV=development and vars are unset)
-# REDIS_HOST=localhost
-# REDIS_PORT=6379
-# REDIS_PASSWORD=
-# REDIS_DATABASE=1
+# Redis (development reads from env, default host 192.168.9.127)
+REDIS_HOST=192.168.9.127
+REDIS_PORT=6379
+REDIS_PASSWORD=
+REDIS_DATABASE=1
 
 # S3 (local development only; production uses EC2 IAM role)
 # AWS_REGION=eu-central-1