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/handlers.rs

@@ -0,0 +1,55 @@
+//! 计费 HTTP 处理器
+
+use axum::{
+    extract::{Extension, Path, State},
+    Json,
+};
+
+use crate::auth::types::AuthContext;
+use crate::error::SaasResult;
+use crate::state::AppState;
+use super::service;
+use super::types::*;
+
+/// GET /api/v1/billing/plans — 列出所有活跃计划
+pub async fn list_plans(
+    State(state): State<AppState>,
+) -> SaasResult<Json<Vec<BillingPlan>>> {
+    let plans = service::list_plans(&state.db).await?;
+    Ok(Json(plans))
+}
+
+/// GET /api/v1/billing/plans/:id — 获取单个计划详情
+pub async fn get_plan(
+    State(state): State<AppState>,
+    Path(plan_id): Path<String>,
+) -> SaasResult<Json<BillingPlan>> {
+    let plan = service::get_plan(&state.db, &plan_id).await?
+        .ok_or_else(|| crate::error::SaasError::NotFound("计划不存在".into()))?;
+    Ok(Json(plan))
+}
+
+/// GET /api/v1/billing/subscription — 获取当前订阅
+pub async fn get_subscription(
+    State(state): State<AppState>,
+    Extension(ctx): Extension<AuthContext>,
+) -> SaasResult<Json<serde_json::Value>> {
+    let plan = service::get_account_plan(&state.db, &ctx.account_id).await?;
+    let sub = service::get_active_subscription(&state.db, &ctx.account_id).await?;
+    let usage = service::get_or_create_usage(&state.db, &ctx.account_id).await?;
+
+    Ok(Json(serde_json::json!({
+        "plan": plan,
+        "subscription": sub,
+        "usage": usage,
+    })))
+}
+
+/// GET /api/v1/billing/usage — 获取当月用量
+pub async fn get_usage(
+    State(state): State<AppState>,
+    Extension(ctx): Extension<AuthContext>,
+) -> SaasResult<Json<UsageQuota>> {
+    let usage = service::get_or_create_usage(&state.db, &ctx.account_id).await?;
+    Ok(Json(usage))
+}