fix(usage): stop 429s with an activity-dependent poll cadence + manual refresh

The usage monitor polled the undocumented OAuth usage endpoint every 60s and
earned 429s. It now polls every 5 min while any task is Running and every
15 min while idle (usage_poll_interval_active_seconds / _idle_seconds, both
clamped to >= 60; the old single usage_poll_interval_seconds key is gone).

A 429 comes back as UsageRateLimitedException carrying Retry-After and adds
exponential backoff on top, capped at 30 min and never shorter than the normal
cadence; the strike count resets on the first success. The schedule arithmetic
is the pure static UsagePollSchedule.NextDelay.

Since the idle cadence is slow on purpose, WorkerHub.RefreshUsage drives
UsageMonitorService.RefreshNowAsync behind a Refresh now button in the Usage
Monitor modal: an out-of-band poll that pushes the loop's next-due time out so
no double poll follows, with a 10s cooldown so click-spam can't earn a 429.

Staleness now measures against the slower (idle) interval so an idle worker
isn't flagged stale just for not polling.

src/ClaudeDo.Worker/Program.cs

@@ -209,7 +209,10 @@ builder.Services.AddHttpClient<IUsageClient, ClaudeOAuthUsageClient>(client =>
 });
 builder.Services.AddSingleton<IUsageGate, UsageGate>();
 builder.Services.AddSingleton<UsageSnapshotBuilder>();
-builder.Services.AddHostedService<UsageMonitorService>();
+builder.Services.AddSingleton<IRunningTaskProbe, RunningTaskProbe>();
+// Singleton + hosted service (same instance) so WorkerHub.RefreshUsage can drive a manual poll.
+builder.Services.AddSingleton<UsageMonitorService>();
+builder.Services.AddHostedService(sp => sp.GetRequiredService<UsageMonitorService>());
 
 // Loopback-only bind. Firewall is irrelevant for 127.0.0.1.
 builder.WebHost.UseUrls($"http://127.0.0.1:{cfg.SignalRPort}");