feat(worker): expose usage/model-usage hub surface and persist run model

Adds GetUsageSnapshot/GetModelUsage/GetTaskUsage to WorkerHub (backed by a
shared UsageSnapshotBuilder), a UsageUpdated broadcast fired after every
UsageMonitorService poll cycle, and records the resolved model on each
task_runs row so per-model/per-task usage can be reported from history.

src/ClaudeDo.Worker/Usage/UsageMonitorService.cs

@@ -1,4 +1,5 @@
 using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Hub;
 using ClaudeDo.Worker.Usage.Interfaces;
 
 namespace ClaudeDo.Worker.Usage;
@@ -7,6 +8,8 @@ namespace ClaudeDo.Worker.Usage;
 /// Polls <see cref="IUsageClient"/> on <see cref="WorkerConfig.UsagePollIntervalSeconds"/> and keeps
 /// <see cref="UsageState"/> current. Polls once immediately at startup. A failure is logged as a
 /// warning at most once per distinct error message, to avoid log spam on a persistent outage.
+/// Broadcasts <see cref="HubBroadcaster.UsageUpdated"/> after every poll cycle, success or failure,
+/// so the UI can reflect a stale/blocked state as soon as it happens.
 /// </summary>
 public sealed class UsageMonitorService : BackgroundService
 {
@@ -14,15 +17,20 @@ public sealed class UsageMonitorService : BackgroundService
     private readonly UsageState _state;
     private readonly WorkerConfig _config;
     private readonly ILogger<UsageMonitorService> _logger;
+    private readonly UsageSnapshotBuilder _snapshotBuilder;
+    private readonly HubBroadcaster _broadcaster;
     private string? _lastLoggedError;
 
     public UsageMonitorService(
-        IUsageClient client, UsageState state, WorkerConfig config, ILogger<UsageMonitorService> logger)
+        IUsageClient client, UsageState state, WorkerConfig config, ILogger<UsageMonitorService> logger,
+        UsageSnapshotBuilder snapshotBuilder, HubBroadcaster broadcaster)
     {
         _client = client;
         _state = state;
         _config = config;
         _logger = logger;
+        _snapshotBuilder = snapshotBuilder;
+        _broadcaster = broadcaster;
     }
 
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
@@ -64,5 +72,8 @@ public sealed class UsageMonitorService : BackgroundService
                 _lastLoggedError = ex.Message;
             }
         }
+
+        var dto = await _snapshotBuilder.BuildAsync(ct);
+        await _broadcaster.UsageUpdated(dto);
     }
 }