fix(notifications): list sort index + CAS operator Retry/Discard (P5-3, P5-4)

P5-3: add IX_Notifications_Created_NotificationId (CreatedAt DESC,
NotificationId DESC) — the Central UI Notification Outbox list page sorts on
exactly that pair, but no existing index leads unconditionally on CreatedAt,
so an unfiltered/lightly filtered page fell back to a full clustered-index
scan + sort on every COUNT + page request. Mirrors the Medium wave's
IX_SiteCalls_Created_TrackedOperationId migration + SQL twin pattern
faithfully, including the handwritten docs/plans/sql script.

P5-4: operator Retry/Discard was read-check-write keyed on NotificationId
only — a detached PipeTo task interleaving with the dispatcher's own write
(or a second concurrent operator action) could land between the read and
the write and get silently clobbered by the unconditional UpdateAsync,
stamping a Delivered row Discarded with stale fields. Adds
INotificationOutboxRepository.TryUpdateFromParkedAsync, a single-statement
compare-and-swap (WHERE Status == Parked) mirroring
SecuredWriteRepository.TryMarkApprovedAsync's CAS shape. NotificationOutboxActor's
RetryAsync/DiscardAsync now use the CAS write and, on a lost race, re-read
to report "notification already progressed to <Status>" instead of silently
overwriting whatever is there now.

Proof of failure: reverting only the actor's CAS usage while keeping the
repository addition reproduces the silent-overwrite bug — 7 tests fail,
including two new deterministic (non-sleep) race-proof tests that mock the
exact GetByIdAsync -> lost-CAS -> re-read sequence the finding describes.
P5-3 is perf-only; verified via migration/SQL-twin parity instead (no
runtime proof-of-failure is meaningful for an index).

tests/ZB.MOM.WW.ScadaBridge.NotificationOutbox.Tests/NotificationOutboxActorRetryEmissionTests.cs

@@ -147,7 +147,7 @@ public class NotificationOutboxActorRetryEmissionTests : TestKit
         ExpectMsg<RetryNotificationResponse>(r => r.Success);
         AwaitAssert(() =>
         {
-            _outboxRepository.Received(1).UpdateAsync(
+            _outboxRepository.Received(1).TryUpdateFromParkedAsync(
                 Arg.Is<Notification>(n => n.Status == NotificationStatus.Pending),
                 Arg.Any<CancellationToken>());
         });