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.

tests/ClaudeDo.Worker.Tests/Usage/UsageSnapshotBuilderTests.cs

@@ -96,7 +96,7 @@ public sealed class UsageSnapshotBuilderTests : IDisposable
     public async Task Snapshot_older_than_4x_poll_interval_is_stale()
     {
         await SetThresholdsAsync(80, 90);
-        var cfg = new WorkerConfig { UsagePollIntervalSeconds = 60 };
+        var cfg = new WorkerConfig { UsagePollIntervalActiveSeconds = 60, UsagePollIntervalIdleSeconds = 60 };
         var state = new UsageState();
         state.ReportSuccess(new UsageSnapshot(
             new UsageBucket(10, null), null, Array.Empty<UsageLimitRow>(),
@@ -113,7 +113,7 @@ public sealed class UsageSnapshotBuilderTests : IDisposable
     public async Task Fresh_snapshot_is_not_stale()
     {
         await SetThresholdsAsync(80, 90);
-        var cfg = new WorkerConfig { UsagePollIntervalSeconds = 60 };
+        var cfg = new WorkerConfig { UsagePollIntervalActiveSeconds = 60, UsagePollIntervalIdleSeconds = 60 };
         var state = new UsageState();
         state.ReportSuccess(new UsageSnapshot(
             new UsageBucket(10, null), null, Array.Empty<UsageLimitRow>(),