feat(health): oldest-parked-message age on the site health report (parked-retention aging signal)

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardStorage.cs

@@ -663,6 +663,26 @@ public class StoreAndForwardStorage
         return Convert.ToInt32(result);
     }
 
+    /// <summary>
+    /// Gets the <c>created_at</c> of the oldest parked message, or <c>null</c> when
+    /// no rows are parked. Backs the parked-retention aging signal on the site health
+    /// report: parked rows persist until an operator acts, so their <i>age</i> — not
+    /// just their count — is what tells an operator a forgotten site's backlog is
+    /// growing stale.
+    /// </summary>
+    /// <returns>A task that resolves to the oldest parked row's creation time, or <c>null</c> if none are parked.</returns>
+    public async Task<DateTimeOffset?> GetOldestParkedCreatedAtAsync()
+    {
+        await using var conn = await OpenConnectionAsync();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = "SELECT MIN(created_at) FROM sf_messages WHERE status = @parked";
+        cmd.Parameters.AddWithValue("@parked", (int)StoreAndForwardMessageStatus.Parked);
+        var result = await cmd.ExecuteScalarAsync();
+        return result is null or DBNull
+            ? null
+            : DateTimeOffset.Parse((string)result);
+    }
+
     /// <summary>
     /// Gets total message count by status.
     /// </summary>