refactor(quota): canonical <entity>_<action> taxonomy for quota events

The quota-attribution action keys were inconsistent (mixed verbs, ad-hoc names,
English-only labels). Introduce a QuotaAction constants holder (one source of
truth), rename every attribution call site to it, and add migration 0020 to
rename historical quota_events.action values. The display label now resolves
from i18n (quotaActions.<key>, EN/HU/DE) instead of a hard-coded English map.
Scheduler job ids and progress phase labels are a separate namespace and are
left untouched.

backend/app/sync/playlists.py

@@ -387,7 +387,7 @@ def sync_all_playlists(db: Session) -> dict:
             progress.report(i, len(users), "playlist_sync")
             continue
         try:
-            with quota.attribute(user.id, "playlist_sync"):
+            with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
                 total += sync_user_playlists(db, user).get("synced", 0)
         except YouTubeError:
             db.rollback()