Caught live by the new lookup timing ("Genius track lookup took 242.4s"):
the 429 handler slept the backoff (30/60/120s) in the CALLING thread and then
re-raised anyway — the import pipeline waited 2x120s per track for lookups
that still failed. Worse, the pre-flight backoff wait also slept while
HOLDING the global Genius API lock, so every other Genius caller queued
serially behind the nap.
Now the backoff is a gate: a 429 opens a 30s->60s->120s window and re-raises
immediately; any call inside the window raises GeniusRateLimitedError on the
spot. The error subclasses requests.RequestException, so every existing
caller (the import's source lookups catch RequestException and skip; the
worker's per-item guards) already handles it as a one-line skip — lyrics and
Genius tags are garnish, nothing is allowed to WAIT for them.
Tests: backoff window fails fast (<0.5s vs the old full-window sleep), a 429
opens and escalates the gate without sleeping, the error is a
RequestException (the no-call-site-changes hinge), success decays the gate.
