Store: per-host backoff after 403/429 honouring Retry-After

A host that answers 403/429 is left alone until its Retry-After (or GitHub's
rate-limit reset; default 10 minutes). Meanwhile cached data is served, or the
source reports 'limited' with its own name, e.g. 'GitHub is limiting requests;
try again in 10 minutes'. Covers _web reads/downloads and F-Droid fetches.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/apk_sources/itch.py

@@ -54,7 +54,7 @@ def search(source, query, limit=50):
     if tag not in FEEDS:
         raise SourceError('Unsupported itch.io feed')
     url = 'https://itch.io/games/free/platform-android/tag-' + tag + '.xml'
-    for entry in _parse(source, _web.read(url, ('itch.io',))):
+    for entry in _parse(source, _web.read(url, ('itch.io',), name='itch.io')):
         entries.setdefault(entry['id'], entry)
     words = query.lower().split()
     return [e for e in entries.values() if all(w in (e['name'] + ' ' + e['summary']).lower()