feat(saas): add billing infrastructure — tables, types, service, handlers

B1.1 Billing database:
- 5 tables: billing_plans, billing_subscriptions, billing_invoices,
  billing_payments, billing_usage_quotas
- Seed data: Free(¥0)/Pro(¥49)/Team(¥199) plans
- JSONB limits for flexible plan configuration

Billing module (crates/zclaw-saas/src/billing/):
- types.rs: BillingPlan, Subscription, Invoice, Payment, UsageQuota
- service.rs: plan CRUD, subscription lookup, usage tracking, quota check
- handlers.rs: REST API (plans list/detail, subscription, usage)
- mod.rs: routes registered at /api/v1/billing/*

Cargo.toml: added chrono feature to sqlx for DateTime<Utc> support

crates/zclaw-saas/src/billing/mod.rs

@@ -0,0 +1,15 @@
+//! 计费模块 — 计划管理、订阅、用量配额
+
+pub mod types;
+pub mod service;
+pub mod handlers;
+
+use axum::routing::get;
+
+pub fn routes() -> axum::Router<crate::state::AppState> {
+    axum::Router::new()
+        .route("/api/v1/billing/plans", get(handlers::list_plans))
+        .route("/api/v1/billing/plans/{id}", get(handlers::get_plan))
+        .route("/api/v1/billing/subscription", get(handlers::get_subscription))
+        .route("/api/v1/billing/usage", get(handlers::get_usage))
+}