Back off from e621 rate limits and pace requests more conservatively

e621 intermittently answers 429 to the IQDB endpoint; browsers hide that
status behind CORS ('Access-Control-Allow-Origin missing'), so the SPA
cannot read it. Treat every network-level failure as a possible rate limit
and pause all e621 traffic for a minute. The cooldown is shared through
localStorage so extra tabs respect it, requests are spaced 1.5s apart
instead of 1s, user-cancelled requests do not trigger a cooldown, and the
upload queue waits the cooldown out with a countdown instead of looking
stuck.

Server side: the per-process e621 gap goes from 0.5s to 1s so two gunicorn
workers cannot together exceed e621's 2/s hard limit.

backend/apps/library/e621.py

@@ -13,7 +13,10 @@ import time
 import requests
 from django.conf import settings
 
-REQUEST_INTERVAL = 0.5  # seconds between requests, per process
+# Seconds between requests, per process. e621 allows 2/s hard and 1/s
+# sustained; each gunicorn worker throttles on its own, so leave enough
+# headroom that combined traffic does not trip the limit.
+REQUEST_INTERVAL = 1.0
 
 
 class E621Error(Exception):