fix(v2): 币种错误映射 409/503(不再误导 500 重试)+ DailyLimit 量纲注释

- handler: ErrCurrencyMismatch → 409, ErrNoSettleCurrency → 503
- test: TestV2RetryCurrencyMismatch409 验证重试不同币种渠道返回 409
- config: DailyLimit 添加量纲注释(minor units)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

config/config.go

@@ -27,10 +27,12 @@ type Config struct {
 // 只存 CredentialEnvPrefix，真值由 internal/accounts.Registry 运行时从环境变量
 // `<CredentialEnvPrefix>_<KEY>` 读取。
 type AccountConfig struct {
-	AccountID           string `mapstructure:"account_id"`
-	Channel             string `mapstructure:"channel"`
-	Weight              int    `mapstructure:"weight"`
-	Enabled             bool   `mapstructure:"enabled"`
+	AccountID string `mapstructure:"account_id"`
+	Channel   string `mapstructure:"channel"`
+	Weight    int    `mapstructure:"weight"`
+	Enabled   bool   `mapstructure:"enabled"`
+	// DailyLimit 单位为该账户所属渠道结算币种的最小单位(minor units)——渠道币种由 Capabilities().SettleCurrencies[0] 决定;
+	// 跨币种账户的限额刻度互不可比,运营配置时按各自币种填。
 	DailyLimit          int64  `mapstructure:"daily_limit"`
 	Region              string `mapstructure:"region"`
 	Subject             string `mapstructure:"subject"`