feat(scaudit): SiteCallAuditActor minimum surface (#22, #23 M3)

Bundle C of Audit Log #23 M3.

Adds the ScadaLink.SiteCallAudit project + matching tests project, mirroring
the ScadaLink.AuditLog scaffolding pattern (net10.0, central package
management, InternalsVisibleTo to the tests assembly).

SiteCallAuditActor is the central singleton entry point for Site Call Audit
(#22): it receives UpsertSiteCallCommand and persists the SiteCall via
ISiteCallAuditRepository.UpsertAsync (monotonic, idempotent — out-of-order
or duplicate updates are silent no-ops at the repo). Audit-write failures
NEVER abort the user-facing action (CLAUDE.md): repository throws are
caught + logged, the actor replies Accepted=false, and the singleton stays
alive (Resume supervisor strategy as defence in depth).

Two constructors mirror AuditLogIngestActor:
- IServiceProvider production constructor resolves the scoped EF repository
  from a fresh DI scope per message.
- ISiteCallAuditRepository test constructor injects a concrete repository so
  the TestKit tests exercise the real monotonic-upsert SQL end to end.

UpsertSiteCallCommand + UpsertSiteCallReply live in ScadaLink.Commons (same
home as IngestAuditEventsCommand) so Bundle D's gRPC server can construct
them without taking a project reference on the actor's host project.

AddSiteCallAudit() is a placeholder for symmetry with AddAuditLog /
AddNotificationOutbox; Bundle F will populate it with the actor's Props
factory + options bindings.

Tests (Akka.TestKit.Xunit2 + MsSqlMigrationFixture via project ref to
ScadaLink.ConfigurationDatabase.Tests, mirroring Bundle D2):
- Receive_UpsertSiteCallCommand_Persists_Replies_Accepted
- Receive_DuplicateUpsert_OlderStatus_NoOp_StillRepliesAccepted (idempotency)
- Receive_RepoThrowsTransient_RepliesAccepted_False_ActorStaysAlive

Reconciliation, KPIs, and the central->site Retry/Discard relay are
deferred per CLAUDE.md scope discipline.

ScadaLink.slnx updated to include both new projects.

All 3 new tests pass against the running infra/mssql container; full suite
(2683 tests across 27 projects) passes with no regressions.

src/ScadaLink.Commons/Messages/Audit/UpsertSiteCallCommand.cs

@@ -0,0 +1,19 @@
+using ScadaLink.Commons.Entities.Audit;
+
+namespace ScadaLink.Commons.Messages.Audit;
+
+/// <summary>
+/// Akka message sent to the central <c>SiteCallAuditActor</c> (Site Call Audit
+/// #22, Audit Log #23 M3 Bundle C) carrying one <see cref="SiteCall"/> row to
+/// be persisted via <c>ISiteCallAuditRepository.UpsertAsync</c>. The repository
+/// performs an insert-if-not-exists then monotonic update — duplicate gRPC
+/// packets and reconciliation pulls can both feed the actor without rolling
+/// state back.
+/// </summary>
+/// <remarks>
+/// Lives in <c>ScadaLink.Commons</c> rather than <c>ScadaLink.SiteCallAudit</c>
+/// so the gRPC server in <c>ScadaLink.Communication</c> can construct it
+/// without taking a project reference on the actor's host project (Bundle D
+/// adds the IngestCachedTelemetry RPC that will Tell this command).
+/// </remarks>
+public sealed record UpsertSiteCallCommand(SiteCall SiteCall);

src/ScadaLink.Commons/Messages/Audit/UpsertSiteCallReply.cs

@@ -0,0 +1,14 @@
+using ScadaLink.Commons.Types;
+
+namespace ScadaLink.Commons.Messages.Audit;
+
+/// <summary>
+/// Reply from the central <c>SiteCallAuditActor</c> for an
+/// <see cref="UpsertSiteCallCommand"/>. <see cref="Accepted"/> is <c>true</c>
+/// when the upsert reached the repository without throwing (including the
+/// monotonic-no-op case where the stored status' rank wins) and <c>false</c>
+/// when persistence raised an exception. The actor itself stays alive in
+/// either case — audit-write failures must NEVER abort the user-facing action
+/// (Audit Log #23 §13).
+/// </summary>
+public sealed record UpsertSiteCallReply(TrackedOperationId TrackedOperationId, bool Accepted);