feat(reconcile): site-reconcile messages + expected-set/stage-if-absent repo

- Commons: ReconcileSiteRequest / ReconcileSiteResponse / ReconcileGapItem
  message contracts (site→central ClusterClient on startup; central reply with
  gap fetch tokens + orphan list + base URL).
- Commons: ExpectedDeployment projection record (Commons/Types/Deployment/),
  lightweight join of DeployedConfigSnapshot + Instance (no ConfigJson).
- IDeploymentManagerRepository: GetExpectedDeploymentsForSiteAsync (inner-join
  query returning deployed set for a site, excluding snapshot-less instances) +
  StagePendingIfAbsentAsync (insert-if-absent, self-contained save, returns bool;
  does NOT supersede — an existing pending row signals in-flight delivery).
- DeploymentManagerRepository: implement both methods; StagePendingIfAbsent
  commits internally (matches PurgeExpiredPendingDeployments convention).
- ReconcileRepositoryTests: 4 tests covering expected-set filter/IsEnabled/
  cross-site isolation, empty-site, stage-absent (true + row retrievable),
  stage-present (false + existing row unchanged); all pass.

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Repositories/IDeploymentManagerRepository.cs

@@ -1,5 +1,6 @@
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Deployment;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Instances;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Deployment;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
 
 namespace ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
@@ -174,6 +175,42 @@ public interface IDeploymentManagerRepository
     /// <returns>The number of pending deployments purged.</returns>
     Task<int> PurgeExpiredPendingDeploymentsAsync(DateTimeOffset nowUtc, CancellationToken cancellationToken = default);
 
+    // Startup reconciliation: expected-set query and insert-if-absent staging
+
+    /// <summary>
+    /// Returns the set of instances that central considers deployed for the given site — the join of
+    /// <c>DeployedConfigSnapshot</c> (by InstanceId) with <c>Instance</c> (where SiteId == siteId).
+    /// Instances without a snapshot are excluded. No <c>ConfigurationJson</c> is loaded; the full
+    /// config is fetched on demand via the HTTP endpoint using a freshly-minted token from
+    /// <see cref="StagePendingIfAbsentAsync"/>.
+    /// </summary>
+    /// <param name="siteId">The site primary key to filter on.</param>
+    /// <param name="cancellationToken">A cancellation token that can be used to cancel the operation.</param>
+    /// <returns>All deployed instances for the site, as lightweight <see cref="ExpectedDeployment"/> projections.</returns>
+    Task<IReadOnlyList<ExpectedDeployment>> GetExpectedDeploymentsForSiteAsync(int siteId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Inserts a <see cref="PendingDeployment"/> for the instance ONLY IF no pending row already
+    /// exists for that <c>InstanceId</c>. An existing pending row signals an in-flight deploy that
+    /// is already delivering to the node — do NOT supersede it (contrast with
+    /// <see cref="AddPendingDeploymentAsync"/>, which supersedes). Returns <c>true</c> if the row
+    /// was staged, <c>false</c> if an existing row was found and left untouched.
+    /// This method is self-contained: it commits its own save and returns a meaningful result,
+    /// matching the convention of <see cref="PurgeExpiredPendingDeploymentsAsync"/>.
+    /// </summary>
+    /// <param name="instanceId">The instance to stage the pending deployment for.</param>
+    /// <param name="deploymentId">The deployment ID (fetch key).</param>
+    /// <param name="revisionHash">Revision hash of the flattened configuration.</param>
+    /// <param name="configurationJson">JSON-serialized flattened configuration.</param>
+    /// <param name="token">Short-TTL fetch token the site node will present to the HTTP endpoint.</param>
+    /// <param name="createdAtUtc">UTC timestamp for this pending row.</param>
+    /// <param name="expiresAtUtc">UTC expiry after which the token is no longer valid.</param>
+    /// <param name="cancellationToken">A cancellation token that can be used to cancel the operation.</param>
+    /// <returns><c>true</c> if staged; <c>false</c> if a pending row already existed and was left unchanged.</returns>
+    Task<bool> StagePendingIfAbsentAsync(int instanceId, string deploymentId, string revisionHash,
+        string configurationJson, string token, DateTimeOffset createdAtUtc, DateTimeOffset expiresAtUtc,
+        CancellationToken cancellationToken = default);
+
     // Instance lookups for deployment pipeline
     /// <summary>
     /// Gets an instance by ID.