feat(#253 PR2): optimistic-concurrency on schedule-items aggregate

Wire the frozen #253 ETag/If-Match/412 recipe onto ProgramSchedule /
schedule-items, keeping the PR#258 positional in-place reconcile intact.

Backend:
- ReplaceProgramScheduleItems command gains Option<int> ExpectedVersion;
  ReplaceScheduleItemsRequest.ToCommand threads it.
- Handler: standalone CheckVersion Either AFTER validation (so 412 isn't
  flattened to 422), unconditional Version++ before save, guarded save via
  SaveChangesWithConcurrencyGuard, and 412 propagated without running the
  post-save reload/enqueue.
- ProgramScheduleViewModel + Mapper carry Version.
- ScheduleController: GET /items emits ETag; PUT /items parses If-Match
  (malformed -> 400), threads ExpectedVersion, re-queries for the new ETag,
  and advertises 400/412.
- Sibling config-writers (Add/Delete item, Update schedule) bump Version.

Frontend:
- schedules.ts: getScheduleItemsWithMeta + replaceScheduleItems(ifMatch)
  returning ResponseWithMeta.
- SchedulesScreen: etagRef threaded through the #242 dirty-guard (set from
  load + every successful save); 412 opens a conflict ConfirmDialog whose
  Reload discards the draft and re-runs loadItems.

Tests: handler concurrency suite (stale->412 no mutation + fill-group state
untouched, match/absent success+bump, no-op still bumps, racing save->412);
controller ETag/If-Match/412 cases; SchedulesScreen 412-conflict-dialog test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Tests/Controllers/PlayoutControllerTests.cs

@@ -1247,7 +1247,7 @@ public class PlayoutControllerTests
         new(0, programScheduleId, [], [], [], false, 1, 1, null, 12, 31, null);
 
     private static ProgramScheduleViewModel MakeScheduleVm(int id) =>
-        new(id, $"Schedule {id}", false, false, false, false, FixedStartTimeBehavior.Strict);
+        new(id, $"Schedule {id}", false, false, false, false, FixedStartTimeBehavior.Strict, 0);
 
     private static TemplateViewModel MakeTemplateViewModel(int id) =>
         new(id, 1, "Group", $"Template {id}");