feat(timeline): emit application lifecycle events

The timeline could interpret InterviewScheduled, InterviewCompleted,
OfferReceived and FollowUpCompleted, but only StatusChanged and FollowUpSet were
ever written, so those branches never rendered.

Events are now derived from the status TRANSITION in one shared emitter rather
than at each call site, so the two status-change boundaries in
JobApplicationsController cannot drift apart and a third would get the behaviour
for free. Both boundaries now call it instead of hand-writing the StatusChanged
block.

Deriving from the transition rather than the resulting state is what prevents
duplicates: one user action produces at most one lifecycle event, re-saving an
unchanged status produces none, and reaching an offer twice records it once.
Moving an application backwards is treated as a correction, not a completed
interview, so only a forward move out of an interview stage counts. An
Interview to Offer move reports the offer, which is the thing the user cares
about.

Completing a follow-up checklist item emits FollowUpCompleted, guarded on the
same transition rule so re-saving a done item stays silent. The task itself
remains a checklist item — this only records that it happened.

No new history store: every event is a JobEvent row, which stays the single
source of application history.

393 backend tests pass, including timeline rendering of the emitted events.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi/Services/ApplicationChecklistService.cs

@@ -193,9 +193,18 @@ public sealed class ApplicationChecklistService : IApplicationChecklistService
             if (ChecklistCategories.IsValid(input.Category)) item.Category = input.Category!;
         }
 
+        var wasDone = item.Status == ChecklistStatuses.Done;
+
         if (ChecklistStatuses.IsValid(input.Status)) Stamp(item, input.Status!);
         else Stamp(item);
 
+        // Ticking off a follow-up task is a real lifecycle moment, so the timeline records it. Guarded
+        // on the transition, so re-saving an already-done item does not emit a second event.
+        if (!wasDone && item.Status == ChecklistStatuses.Done && item.Category == ChecklistCategories.FollowUp)
+        {
+            JobLifecycleEvents.RecordFollowUpCompleted(_db, jobApplicationId, item.Title);
+        }
+
         await _db.SaveChangesAsync(ct);
         return Project(item);
     }