feat(billing): 完成异步结算与请求执行闭环

统一任务成功、Attempt 与结算 Outbox 的事务边界，增加多实例安全结算、人工复核、请求幂等与执行租约。钱包决策使用九位精确金额，并通过审计保护约束保留流水事实；同时补充管理接口、指标与 PostgreSQL 集成测试。

.env.example

@@ -23,6 +23,12 @@ CONFIG_JWT_SECRET=this is a very secret secret
 # - hybrid: both sources are accepted and separated by gateway_users.source.
 IDENTITY_MODE=hybrid
 
+# Billing engine rollout mode:
+# - observe: keep legacy billing decisions and compare effective-pricing-v2 in logs.
+# - enforce: require v2 pricing, reserve the candidate maximum, then settle asynchronously.
+# - hold: reject new production generation before any upstream request; existing settlements continue.
+BILLING_ENGINE_MODE=observe
+
 # Unified identity business settings are managed in System Settings > Unified
 # Identity. Deployment only supplies the SecretStore and infrastructure timing.
 AI_GATEWAY_PUBLIC_BASE_URL=http://localhost:8088