feat(S2-b): connect durable MQTT sync to capture workflow

FieldLogger/Services/Sync/IMqttSyncService.cs

@@ -3,25 +3,30 @@ using FieldLogger.Models;
 namespace FieldLogger.Services.Sync;
 
 /// <summary>
-/// Placeholder for the future MQTT synchronization layer:
-/// - subscribe to receive jobs configured on the server
-/// - publish logged points as they are captured
-/// - reconcile the Synced flags on <see cref="Job"/> and <see cref="LoggedPoint"/>
-/// Planned implementation: MQTTnet client against the configured broker.
+/// Durable app MQTT synchronization. Captures are persisted locally before this service queues
+/// them, and queue rows are released only by an application-level ACCEPTED/DUPLICATE ack.
 /// </summary>
 public interface IMqttSyncService
 {
     bool IsConnected { get; }
+    string Status { get; }
+    event EventHandler<string>? StatusChanged;
+    event EventHandler<PointSyncFailureEventArgs>? PointRejected;
+    Task StartAsync(CancellationToken cancellationToken = default);
+    Task StopAsync();
     Task ConnectAsync(CancellationToken cancellationToken = default);
     Task DisconnectAsync();
     Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default);
 }
 
-/// <summary>No-op stand-in until the MQTT backend exists.</summary>
-public sealed class NullMqttSyncService : IMqttSyncService
+public sealed class PointSyncFailureEventArgs : EventArgs
 {
-    public bool IsConnected => false;
-    public Task ConnectAsync(CancellationToken cancellationToken = default) => Task.CompletedTask;
-    public Task DisconnectAsync() => Task.CompletedTask;
-    public Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default) => Task.CompletedTask;
+    public LoggedPoint Point { get; }
+    public string ReasonCode { get; }
+
+    public PointSyncFailureEventArgs(LoggedPoint point, string reasonCode)
+    {
+        Point = point;
+        ReasonCode = reasonCode;
+    }
 }