feat(health): wire up NodeHostname, ConnectionEndpoint, TagQuality, ParkedMessageCount collectors

- AkkaHostedService: SetNodeHostname from NodeOptions
- DataConnectionActor: UpdateConnectionEndpoint on state transitions,
  track per-tag quality counts and UpdateTagQuality on value changes
- HealthReportSender: query StoreAndForwardStorage for parked message count
- StoreAndForwardStorage: add GetParkedMessageCountAsync()

src/ScadaLink.StoreAndForward/StoreAndForwardStorage.cs

@@ -297,6 +297,20 @@ public class StoreAndForwardStorage
         return messages.FirstOrDefault();
     }
 
+    /// <summary>
+    /// Gets the count of parked messages (for health reporting).
+    /// </summary>
+    public async Task<int> GetParkedMessageCountAsync()
+    {
+        await using var conn = new SqliteConnection(_connectionString);
+        await conn.OpenAsync();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = "SELECT COUNT(*) FROM sf_messages WHERE status = @parked";
+        cmd.Parameters.AddWithValue("@parked", (int)StoreAndForwardMessageStatus.Parked);
+        var result = await cmd.ExecuteScalarAsync();
+        return Convert.ToInt32(result);
+    }
+
     /// <summary>
     /// Gets total message count by status.
     /// </summary>