fix: a batch the user themselves cancelled is not called a lapse

Picking "Alap" revokes the consent, and a batch still in flight is then answered by the server with
the same 409 a real expiry gets. The 409 arm only checked that the consent session had not changed,
not that this tab was still streaming — so the user's own log gained a "diagnostics level lapsed"
line for a level they had just turned off themselves, plus a pointless `me` refetch. The same guard
also stops a lapse this tab has already announced being announced a second time.

Measured in Chrome by holding the POST at the network layer, switching to Alap and only then
answering 409: the line appears without the guard and not with it.

Three tests that named the session counter never reached it — they settled the refusal while the
stream was already stopped, where `unsubscribe` does the work, so the counter could be deleted from
both arms with the suite green. The refusal now settles after the re-raise, which is the ordering it
exists for. Same for the queue trim on the 429 path: the old test unshifted a batch into an empty
queue, so the trim was a no-op and only the subscribe path was ever measured.

frontend/src/lib/diagnostics.ts

@@ -128,8 +128,17 @@ async function flush(): Promise<void> {
         queue.unshift(...batch);
         keepNewest();
       }
-    } else if (err instanceof HttpError && err.status === 409 && takenIn === session) {
-      lapse(); // the server has it off: withdrawn in another tab, or lapsed by its clock
+    } else if (
+      err instanceof HttpError &&
+      err.status === 409 &&
+      takenIn === session &&
+      unsubscribe
+    ) {
+      // The server has it off: withdrawn in another tab, or lapsed by its clock. `unsubscribe`
+      // means THIS tab does not already know — without it, the 409 answering a batch that was in
+      // flight when the user picked Alap writes "lapsed" into their own log for a level they
+      // revoked, and a lapse this tab has already announced gets announced a second time.
+      lapse();
     }
   } finally {
     inFlight = false;