fix(auditlog)+fix(siteeventlog): complete audit write tasks only after commit; run event-log retention on standby (arch-review 2026-08-19 P4-2 + P4-4)

P4-2: SqliteAuditWriter.FlushBatch completed each pending row's Task
(TrySetResult) inside the per-row insert loop, before transaction.Commit()
ran. A later failure in the same batch (another row's insert throwing, or
the commit itself failing) rolled the whole transaction back, but the
earlier rows' Tasks had already completed successfully -- the caller and
FallbackAuditWriter (whose ring-buffer fallback exists precisely for this
outage) both saw false success while the rows were actually gone from the
database; the outer catch's TrySetException became a silent no-op against
an already-completed TaskCompletionSource. Every row's Completion is now
deferred and only resolved (TrySetResult for the whole batch, or
TrySetException for the whole batch) after transaction.Commit() succeeds
or the transaction rolls back, mirroring SiteEventLogger.FlushBatch.

P4-4: EventLogPurgeService gated the daily site_events purge on the
active-node check, early-exiting on the standby, on the stale premise that
"the standby receives no writes". DataConnectionActor runs on both site
nodes and writes site_events regardless of active/standby status (register
rows 89/90/97), so an unreplicated standby (the rig's site-b/site-c
posture) grew its site_events file forever with no retention and no cap.
The gate is removed; every node now purges its own file every tick. Safe
on a replicated pair because the retention/cap deletes are CDC-captured
and converge under LWW regardless of which node issues them; required on
an unreplicated pair because each node's file is independent. The
active-node check is retained only to annotate the purge tick's log line.

Tests: new SqliteAuditWriterCommitFailureTests forces a genuine
non-swallowed mid-batch INSERT failure via a real SQLite trigger + custom
scalar function on the writer's own connection (reflection seam), proving
(a) a row whose own insert succeeded still faults when a later row in the
same batch fails, and (b) FallbackAuditWriter rings every faulted row and
increments the failure counter for each -- both assertions fail against
the pre-fix code (proof-of-failure verified by temporarily reverting the
fix). EventLogPurgeServiceTests' standby case is inverted to assert the
purge now runs (not skips) on a standby node.

src/ZB.MOM.WW.ScadaBridge.SiteEventLogging/ServiceCollectionExtensions.cs

@@ -23,13 +23,14 @@ public static class ServiceCollectionExtensions
         services.AddSingleton<ISiteEventLogger>(sp => sp.GetRequiredService<SiteEventLogger>());
         services.AddSingleton<IEventLogQueryService, EventLogQueryService>();
 
-        // The purge service still registers on every host
-        // node, but it consults an optional SiteEventLogActiveNodeCheck on each
-        // tick and early-exits on the standby. The Host registers the real
-        // active-node check on site nodes; tests and non-clustered hosts leave
-        // it unregistered, and the purge defaults to "always run" (the
-        // pre-fix behaviour). Building the service via a factory so the
-        // optional delegate flows from DI rather than the constructor default.
+        // The purge service registers on every host node and purges its own
+        // site_events file on every tick, on every node (arch-review 2026-08-19 P4-4 —
+        // the standby writes site_events too, so it can no longer skip its own purge).
+        // It still consults an optional SiteEventLogActiveNodeCheck, but only to annotate
+        // the tick's log line, not to gate it. The Host registers the real active-node
+        // check on site nodes; tests and non-clustered hosts leave it unregistered, and
+        // the log line just reports every tick as active. Building the service via a
+        // factory so the optional delegate flows from DI rather than the constructor default.
         //
         // SiteEventLogReplicationCheck flows the same way and is likewise Host-supplied:
         // it only selects the wording of the purge log line (whether to add the