The Claude usage poller retried /api/oauth/usage every 60s, but the endpoint
returns 429 with a longer Retry-After window (~93s). Retrying mid-cooldown
re-tripped the limit so the 429s never cleared and usage data never populated.
Record a cooldownUntil from the 429 Retry-After header (5min fallback when
absent) and skip the API until it closes; cleared on success. Dashboard now
shows a "429" indicator instead of a stale value when rate-limited.
Adds regression tests proving the cooldown outlasts the 60s throttle and
releases once the window passes.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
