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/Hub/WorkerHub.cs

@@ -189,6 +189,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
     private readonly Data.Git.GitService? _git;
     private readonly UsageSnapshotBuilder? _usageSnapshotBuilder;
     private readonly ITranscriptUsageReader? _usageReader;
+    private readonly UsageMonitorService? _usageMonitor;
 
     public WorkerHub(
         QueueService queue,
@@ -220,7 +221,8 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         WorktreeManager? worktreeManager = null,
         Data.Git.GitService? git = null,
         UsageSnapshotBuilder? usageSnapshotBuilder = null,
-        ITranscriptUsageReader? usageReader = null)
+        ITranscriptUsageReader? usageReader = null,
+        UsageMonitorService? usageMonitor = null)
     {
         _queue = queue;
         _waker = waker;
@@ -252,6 +254,7 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         _git = git;
         _usageSnapshotBuilder = usageSnapshotBuilder;
         _usageReader = usageReader;
+        _usageMonitor = usageMonitor;
     }
 
     // Persistence boundary for the session_skills JSON-array columns (task/list/global).
@@ -1049,6 +1052,18 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
         return _usageSnapshotBuilder.BuildAsync(Context.ConnectionAborted);
     });
 
+    /// <summary>
+    /// Manual "refresh now" for the usage monitor. Polls the endpoint out of band and returns the
+    /// fresh snapshot; a refresh inside the monitor's cooldown reuses the last poll's result
+    /// instead of risking a 429.
+    /// </summary>
+    public Task<UsageSnapshotDto> RefreshUsage() => HubGuard(() =>
+    {
+        if (_usageMonitor is null)
+            throw new InvalidOperationException("Usage monitor is not configured.");
+        return _usageMonitor.RefreshNowAsync(Context.ConnectionAborted);
+    });
+
     public Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsage(DateOnly from, DateOnly to) => HubGuard(async () =>
     {
         if (_usageReader is null)