fix(cron): merge failureAlert mode/accountId and preserve failureDestination on updates

- Fix mergeCronFailureAlert to merge mode and accountId fields
- Fix mergeCronDelivery to preserve failureDestination on updates
- Fix isSameDeliveryTarget to use 'announce' as default instead of 'none'
  to properly detect duplicates when delivery.mode is undefined

src/cron/delivery.ts

@@ -174,7 +174,7 @@ function isSameDeliveryTarget(
   delivery: CronDelivery,
   failurePlan: CronFailureDeliveryPlan,
 ): boolean {
-  const primaryMode = delivery.mode ?? "none";
+  const primaryMode = delivery.mode ?? "announce";
   if (primaryMode === "none") {
     return false;
   }

src/cron/service/jobs.ts

@@ -668,6 +668,7 @@ function mergeCronDelivery(
     to: existing?.to,
     accountId: existing?.accountId,
     bestEffort: existing?.bestEffort,
+    failureDestination: existing?.failureDestination,
   };
 
   if (typeof patch.mode === "string") {
@@ -685,6 +686,20 @@ function mergeCronDelivery(
   if (typeof patch.bestEffort === "boolean") {
     next.bestEffort = patch.bestEffort;
   }
+  if ("failureDestination" in patch) {
+    if (patch.failureDestination === undefined) {
+      next.failureDestination = undefined;
+    } else {
+      const existingFd = next.failureDestination;
+      const patchFd = patch.failureDestination;
+      next.failureDestination = {
+        channel: patchFd?.channel ?? existingFd?.channel,
+        to: patchFd?.to ?? existingFd?.to,
+        accountId: patchFd?.accountId ?? existingFd?.accountId,
+        mode: patchFd?.mode ?? existingFd?.mode,
+      };
+    }
+  }
 
   return next;
 }
@@ -719,6 +734,14 @@ function mergeCronFailureAlert(
         : -1;
     next.cooldownMs = cooldownMs >= 0 ? Math.floor(cooldownMs) : undefined;
   }
+  if ("mode" in patch) {
+    const mode = typeof patch.mode === "string" ? patch.mode.trim() : "";
+    next.mode = mode === "announce" || mode === "webhook" ? mode : undefined;
+  }
+  if ("accountId" in patch) {
+    const accountId = typeof patch.accountId === "string" ? patch.accountId.trim() : "";
+    next.accountId = accountId ? accountId : undefined;
+  }
 
   return next;
 }