refactor(config): enable environment variable binding for database replica settings

Viper's AutomaticEnv does not bind nested keys when sections are commented out
in YAML. Add explicit BindEnv calls for APP_DATABASE_REPLICA_* variables so
they work when the replica section is not defined in config.yaml. Also bind
optional connection pool parameters (MAX_IDLE_CONNS, MAX_OPEN_CONNS).

configs/config.yaml

@@ -22,9 +22,11 @@ database:
     dbname: with_you
     sslmode: disable
   # 读写分离配置（仅 postgres 模式生效，不配置则所有请求走主库）
-  # 单副本配置（支持环境变量）:
+  # 注意：replica 字段已在 config.go 中显式 BindEnv，环境变量优先于 YAML。
+  # 单副本配置（推荐用环境变量覆盖，无需取消下方注释）:
   #   APP_DATABASE_REPLICA_HOST, APP_DATABASE_REPLICA_PORT, APP_DATABASE_REPLICA_USER,
   #   APP_DATABASE_REPLICA_PASSWORD, APP_DATABASE_REPLICA_DBNAME, APP_DATABASE_REPLICA_SSLMODE
+  #   APP_DATABASE_REPLICA_MAX_IDLE_CONNS, APP_DATABASE_REPLICA_MAX_OPEN_CONNS（可选，缺省回退主库连接池）
   # replica:
   #   host: replica1.internal
   #   port: 5432