Phase 3C: Deployment pipeline & Store-and-Forward engine

Deployment Manager (WP-1–8, WP-16):
- DeploymentService: full pipeline (flatten→validate→send→track→audit)
- OperationLockManager: per-instance concurrency control
- StateTransitionValidator: Enabled/Disabled/NotDeployed transition matrix
- ArtifactDeploymentService: broadcast to all sites with per-site results
- Deployment identity (GUID + revision hash), idempotency, staleness detection
- Instance lifecycle commands (disable/enable/delete) with deduplication

Store-and-Forward (WP-9–15):
- StoreAndForwardStorage: SQLite persistence, 3 categories, no max buffer
- StoreAndForwardService: fixed-interval retry, transient-only buffering, parking
- ReplicationService: async best-effort to standby (fire-and-forget)
- Parked message management (query/retry/discard from central)
- Messages survive instance deletion, S&F drains on disable

620 tests pass (+79 new), zero warnings.

src/ScadaLink.DeploymentManager/IFlatteningPipeline.cs

@@ -0,0 +1,27 @@
+using ScadaLink.Commons.Types;
+using ScadaLink.Commons.Types.Flattening;
+
+namespace ScadaLink.DeploymentManager;
+
+/// <summary>
+/// Abstraction over the TemplateEngine flattening + validation + hashing pipeline.
+/// Used by DeploymentService to obtain a validated, hashed FlattenedConfiguration.
+/// </summary>
+public interface IFlatteningPipeline
+{
+    /// <summary>
+    /// Flattens and validates an instance configuration, returning the configuration,
+    /// revision hash, and validation result.
+    /// </summary>
+    Task<Result<FlatteningPipelineResult>> FlattenAndValidateAsync(
+        int instanceId,
+        CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Result of the flattening pipeline: configuration, hash, and validation.
+/// </summary>
+public record FlatteningPipelineResult(
+    FlattenedConfiguration Configuration,
+    string RevisionHash,
+    ValidationResult Validation);