fix nda

src/background-task.ts

@@ -14,7 +14,13 @@ import { query, queryOne } from './db.js';
 export interface TaskKeys {
   /** idle | running | error:<message> */
   state: string;
-  /** ISO timestamp of the last completed run. */
+  /**
+   * ISO timestamp of the last completed run — written by the **job**, never
+   * by this module. Each job owns its own key and the two sets are disjoint
+   * (`ds_*_refresh_*` vs `ds_*_sync_*`), because `ds_last_refresh_at` is not
+   * merely a display value: it is the boundary the sync uses to decide what
+   * the refresh can no longer reach. See startTask().
+   */
   lastAt: string;
   /** JSON result of the last completed run. */
   lastResult: string;
@@ -71,6 +77,13 @@ export async function clearStaleTask(
 /**
  * Claims the slot and runs `job` detached. `false` means somebody else is
  * already running it — the caller answers 409.
+ *
+ * It deliberately does **not** stamp `keys.lastAt`. A shared helper writing
+ * "this ran at" on every resolved job is how `ds_last_refresh_at` came to move
+ * forward on runs that had walked nothing: the refresh cursor then pointed
+ * past requests that were never mirrored, and nothing downstream could tell
+ * the difference. Each job writes its own `lastAt` at the point where it knows
+ * its work is done and stored.
  */
 export async function startTask<T>(
   keys: TaskKeys,
@@ -84,7 +97,6 @@ export async function startTask<T>(
     try {
       const result = await job();
       await setMeta(keys.lastResult, JSON.stringify(result));
-      await setMeta(keys.lastAt, new Date().toISOString());
       await setMeta(keys.state, 'idle');
     } catch (err) {
       const message = (err as Error).message;