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/Program.cs

@@ -206,8 +206,9 @@ builder.Services.AddHttpClient<IUsageClient, ClaudeOAuthUsageClient>(client =>
 {
     client.Timeout = TimeSpan.FromSeconds(5);
 });
-builder.Services.AddHostedService<UsageMonitorService>();
 builder.Services.AddSingleton<IUsageGate, UsageGate>();
+builder.Services.AddSingleton<UsageSnapshotBuilder>();
+builder.Services.AddHostedService<UsageMonitorService>();
 
 // Loopback-only bind. Firewall is irrelevant for 127.0.0.1.
 builder.WebHost.UseUrls($"http://127.0.0.1:{cfg.SignalRPort}");