feat(commons): add audit telemetry + pull message DTOs (#23)

src/ScadaLink.Commons/Messages/Integration/AuditTelemetryEnvelope.cs

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+using ScadaLink.Commons.Entities.Audit;
+
+namespace ScadaLink.Commons.Messages.Integration;
+
+/// <summary>
+/// Audit Log (#23) telemetry envelope sent from a site to central over gRPC.
+/// At-least-once delivery; central is idempotent on <see cref="AuditEvent.EventId"/>.
+/// See Component-AuditLog.md "Ingestion" for the handoff contract.
+/// </summary>
+public sealed record AuditTelemetryEnvelope(
+    Guid EnvelopeId,
+    string SourceSiteId,
+    IReadOnlyList<AuditEvent> Events);

src/ScadaLink.Commons/Messages/Integration/PullAuditEventsRequest.cs

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+namespace ScadaLink.Commons.Messages.Integration;
+
+/// <summary>
+/// Audit Log (#23) periodic reconciliation pull request: central asks a site for
+/// audit events since the given UTC watermark, up to <paramref name="BatchSize"/>.
+/// Acts as the fallback when streaming telemetry is lost. See Component-AuditLog.md "Ingestion".
+/// </summary>
+public sealed record PullAuditEventsRequest(
+    string SourceSiteId,
+    DateTime SinceUtc,
+    int BatchSize);

src/ScadaLink.Commons/Messages/Integration/PullAuditEventsResponse.cs

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+using ScadaLink.Commons.Entities.Audit;
+
+namespace ScadaLink.Commons.Messages.Integration;
+
+/// <summary>
+/// Audit Log (#23) periodic reconciliation pull response: the next batch of site
+/// audit events plus a <paramref name="MoreAvailable"/> flag signalling the caller
+/// to advance the watermark and pull again. See Component-AuditLog.md "Ingestion".
+/// </summary>
+public sealed record PullAuditEventsResponse(
+    IReadOnlyList<AuditEvent> Events,
+    bool MoreAvailable);