test(analysis): assert Cancelled event is published; correct cron-test rationale (0.87.21)

0.87.13 followup. The Cancelled publish had no asserting test —
mechanical revert of the publish block shipped green. New sqlx test
subscribes before spawn, drives SlowDispatcher into flight, cancels,
asserts both Started and Cancelled frames with breadcrumb fields.
Mutation-confirmed.

Also corrects: cron-test rationale comment (inverted unlock-axis),
Cancelled docstring (self-contradicting clear-triggers sentence).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/analysis/events.rs

@@ -56,10 +56,11 @@ pub enum AnalysisEvent {
     },
     /// The worker cancelled an in-flight dispatch (daemon shutdown or
     /// settings reload toggling analysis off). The lease was released,
-    /// not failed — but the dashboard banner already saw `Started` and
-    /// only clears on `Completed`/`Failed`/`Cancelled`. Without this
-    /// variant the banner stuck on the cancelled page forever (until a
-    /// later page kicked off and overwrote it).
+    /// not failed. The dashboard's "now analyzing" banner clears on
+    /// `Completed`/`Failed`/`Cancelled`; before this variant existed it
+    /// listened only to the first two, so a cancel-mid-dispatch left
+    /// the banner stuck on the cancelled page until a later one
+    /// kicked off and overwrote it.
     Cancelled {
         page_id: Uuid,
         manga_id: Uuid,