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/lib.rs

@@ -25,3 +25,4 @@ pub mod prompt;
 pub mod agent_template;
 pub mod scheduled_task;
 pub mod telemetry;
+pub mod billing;