Translator: languages that fail to download are tried again

The first start tried the default languages once; a DNS failure at that
moment left them missing for good. Languages asked for are now kept as
pending in state.json and retried in the background (30 s, doubling to
15 min) until they are in, across restarts; an install from the admin
panel joins the same queue. A state from before this, with nothing
installed, counts as an unfinished first start. The admin panel shows
what is waiting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/api/test_admin_translation.py

@@ -76,6 +76,7 @@ async def test_unreachable_service_is_reported(
         "reachable": False,
         "installed": [],
         "installing": {},
+        "pending": [],
         "available": [],
         "errors": {},
     }