feat(billing): add usage increment API + wire hand/pipeline execution tracking

Server side:
- POST /api/v1/billing/usage/increment endpoint with dimension whitelist
  (hand_executions, pipeline_runs, relay_requests) and count validation (1-100)
- Returns updated usage quota after increment

Desktop side:
- New saas-billing.ts mixin with incrementUsageDimension() and
  reportUsageFireAndForget() (non-blocking, safe for finally blocks)
- handStore.triggerHand: reports hand_executions after successful run
- PipelinesPanel.handleRunComplete: reports pipeline_runs on completion
- SaaSClient type declarations for new billing methods

Billing pipeline now covers all three dimensions:
  relay_requests  → relay handler (server-side, real-time)
  hand_executions → handStore (client-side, fire-and-forget)
  pipeline_runs   → PipelinesPanel (client-side, fire-and-forget)

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

@@ -56,6 +56,53 @@ pub async fn get_usage(
     Ok(Json(usage))
 }
 
+/// POST /api/v1/billing/usage/increment — 客户端上报用量（Hand/Pipeline 执行后调用）
+///
+/// 请求体: `{ "dimension": "hand_executions" | "pipeline_runs" | "relay_requests", "count": 1 }`
+/// 需要认证 — account_id 从 JWT 提取。
+#[derive(Debug, Deserialize)]
+pub struct IncrementUsageRequest {
+    /// 用量维度：hand_executions / pipeline_runs / relay_requests
+    pub dimension: String,
+    /// 递增数量，默认 1
+    #[serde(default = "default_count")]
+    pub count: i32,
+}
+
+fn default_count() -> i32 { 1 }
+
+pub async fn increment_usage_dimension(
+    State(state): State<AppState>,
+    Extension(ctx): Extension<AuthContext>,
+    Json(req): Json<IncrementUsageRequest>,
+) -> SaasResult<Json<serde_json::Value>> {
+    // 验证维度白名单
+    if !["hand_executions", "pipeline_runs", "relay_requests"].contains(&req.dimension.as_str()) {
+        return Err(SaasError::InvalidInput(
+            format!("无效的用量维度: {}，支持: hand_executions / pipeline_runs / relay_requests", req.dimension)
+        ));
+    }
+
+    // 限制单次递增上限（防滥用）
+    if req.count < 1 || req.count > 100 {
+        return Err(SaasError::InvalidInput(
+            format!("count 必须在 1~100 范围内，得到: {}", req.count)
+        ));
+    }
+
+    for _ in 0..req.count {
+        service::increment_dimension(&state.db, &ctx.account_id, &req.dimension).await?;
+    }
+
+    // 返回更新后的用量
+    let usage = service::get_or_create_usage(&state.db, &ctx.account_id).await?;
+    Ok(Json(serde_json::json!({
+        "dimension": req.dimension,
+        "incremented": req.count,
+        "usage": usage,
+    })))
+}
+
 /// POST /api/v1/billing/payments — 创建支付订单
 pub async fn create_payment(
     State(state): State<AppState>,

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

@@ -13,6 +13,7 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         .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))
+        .route("/api/v1/billing/usage/increment", post(handlers::increment_usage_dimension))
         .route("/api/v1/billing/payments", post(handlers::create_payment))
         .route("/api/v1/billing/payments/{id}", get(handlers::get_payment_status))
         // 支付回调（无需 auth）