Fix ML-143 review: security, worker, and test improvements

Closes ML-143 review findings:
- cast_id/1 uses Integer.parse instead of String.to_integer
- add_to_cart validates against server-side release_groups
- JS.push payload trimmed to {id, format} only
- Oban.insert_all result checked for partial failures
- handle_async success resets @importing? to false
- ImportFromMusicbrainzReleaseGroup gets unique constraint
- Record.parse_datetime/1 made public with safe match
- Workers delegate to Record.parse_datetime/1
- Test coverage: change_format, clear_cart, wishlist single import

priv/gettext/en/LC_MESSAGES/default.po

@@ -2511,3 +2511,8 @@ msgstr ""
 #, elixir-autogen, elixir-format
 msgid "Your cart is empty"
 msgstr ""
+
+#: lib/music_library_web/components/add_record.ex
+#, elixir-autogen, elixir-format
+msgid "Error queuing records for import"
+msgstr ""