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.StoreAndForward/StoreAndForwardMessage.cs

@@ -0,0 +1,49 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.StoreAndForward;
+
+/// <summary>
+/// WP-9: Represents a single store-and-forward message as stored in SQLite.
+/// Maps to the sf_messages table.
+/// </summary>
+public class StoreAndForwardMessage
+{
+    /// <summary>Unique message ID (GUID).</summary>
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>WP-9: Category: ExternalSystem, Notification, or CachedDbWrite.</summary>
+    public StoreAndForwardCategory Category { get; set; }
+
+    /// <summary>Target system name (external system, notification list, or DB connection).</summary>
+    public string Target { get; set; } = string.Empty;
+
+    /// <summary>JSON-serialized payload containing the call details.</summary>
+    public string PayloadJson { get; set; } = string.Empty;
+
+    /// <summary>Number of delivery attempts so far.</summary>
+    public int RetryCount { get; set; }
+
+    /// <summary>Maximum retry attempts before parking (0 = no limit).</summary>
+    public int MaxRetries { get; set; }
+
+    /// <summary>Retry interval in milliseconds.</summary>
+    public long RetryIntervalMs { get; set; }
+
+    /// <summary>When this message was first enqueued.</summary>
+    public DateTimeOffset CreatedAt { get; set; }
+
+    /// <summary>When delivery was last attempted (null if never attempted).</summary>
+    public DateTimeOffset? LastAttemptAt { get; set; }
+
+    /// <summary>Current status of the message.</summary>
+    public StoreAndForwardMessageStatus Status { get; set; }
+
+    /// <summary>Last error message from a failed delivery attempt.</summary>
+    public string? LastError { get; set; }
+
+    /// <summary>
+    /// Instance that originated this message (for S&amp;F-survives-delete behavior).
+    /// WP-13: Messages are NOT cleared when instance is deleted.
+    /// </summary>
+    public string? OriginInstanceName { get; set; }
+}