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/Usage/UsageSnapshotBuilder.cs

@@ -39,7 +39,10 @@ public sealed class UsageSnapshotBuilder
 
         var decision = await _gate.EvaluateAsync(ct);
 
-        var maxAge = TimeSpan.FromSeconds(_cfg.UsagePollIntervalSeconds * 3);
+        // Measured against the *slowest* cadence — the idle interval — so a genuinely idle
+        // worker on its 15-minute heartbeat isn't reported stale just for not polling.
+        var maxAge = TimeSpan.FromSeconds(
+            Math.Max(_cfg.UsagePollIntervalActiveSeconds, _cfg.UsagePollIntervalIdleSeconds) * 3);
         var isStale = snapshot is null || lastError is not null || (DateTime.UtcNow - snapshot.FetchedAtUtc) > maxAge;
 
         var limits = (snapshot?.Limits ?? Array.Empty<UsageLimitRow>())