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.

frontend/src/i18n/locales/de/quotaActions.json

@@ -0,0 +1,16 @@
+{
+  "subscriptions_pull": "Abonnements einlesen",
+  "subscriptions_resync": "Automatische Abo-Resynchronisierung",
+  "playlists_pull": "Playlists einlesen",
+  "playlists_push": "Playlists hochladen",
+  "playlists_delete": "Playlist löschen",
+  "videos_backfill_recent": "Aktuelle Uploads laden",
+  "videos_backfill_full": "Vollständigen Verlauf laden",
+  "videos_enrich": "Video-Anreicherung",
+  "videos_lookup": "Video-Abfrage",
+  "channels_discover": "Kanal-Entdeckung",
+  "channels_subscribe": "Abonnieren",
+  "channels_unsubscribe": "Abo beenden",
+  "maintenance_revalidate": "Wartungs-Neuvalidierung",
+  "other": "Sonstiges"
+}

frontend/src/i18n/locales/en/quotaActions.json

@@ -0,0 +1,16 @@
+{
+  "subscriptions_pull": "Read subscriptions",
+  "subscriptions_resync": "Auto subscription resync",
+  "playlists_pull": "Read playlists",
+  "playlists_push": "Push playlists",
+  "playlists_delete": "Delete playlist",
+  "videos_backfill_recent": "Recent backfill",
+  "videos_backfill_full": "Full-history backfill",
+  "videos_enrich": "Video enrichment",
+  "videos_lookup": "Video lookup",
+  "channels_discover": "Channel discovery",
+  "channels_subscribe": "Subscribe",
+  "channels_unsubscribe": "Unsubscribe",
+  "maintenance_revalidate": "Maintenance re-validation",
+  "other": "Other"
+}

frontend/src/i18n/locales/hu/quotaActions.json

@@ -0,0 +1,16 @@
+{
+  "subscriptions_pull": "Feliratkozások beolvasása",
+  "subscriptions_resync": "Automatikus feliratkozás-újraszinkron",
+  "playlists_pull": "Lejátszási listák beolvasása",
+  "playlists_push": "Lejátszási listák feltöltése",
+  "playlists_delete": "Lejátszási lista törlése",
+  "videos_backfill_recent": "Friss feltöltések betöltése",
+  "videos_backfill_full": "Teljes előzmény betöltése",
+  "videos_enrich": "Videó-gazdagítás",
+  "videos_lookup": "Videó-lekérdezés",
+  "channels_discover": "Csatorna-felfedezés",
+  "channels_subscribe": "Feliratkozás",
+  "channels_unsubscribe": "Leiratkozás",
+  "maintenance_revalidate": "Karbantartó újraellenőrzés",
+  "other": "Egyéb"
+}

frontend/src/lib/format.ts

@@ -44,18 +44,11 @@ export function formatDuration(sec: number | null): string {
   return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
 }
 
-const QUOTA_ACTION_LABELS: Record<string, string> = {
-  sync_subscriptions: "Sync subscriptions",
-  backfill_recent: "Recent backfill",
-  backfill_deep: "Full-history backfill",
-  enrich: "Enrichment",
-  unsubscribe: "Unsubscribe",
-  subscription_resync: "Auto subscription resync",
-  api: "Other",
-};
-
+// Human label for a canonical quota-action key (see backend app.quota.QuotaAction). Resolved
+// from i18n (quotaActions.<key>) so it's translated in all UI languages; falls back to the raw
+// key for any unmapped/legacy value.
 export function quotaActionLabel(action: string): string {
-  return QUOTA_ACTION_LABELS[action] ?? action;
+  return i18n.t(`quotaActions.${action}`, { defaultValue: action });
 }
 
 /** Coarse, human ETA for a remaining duration in seconds ("~3 hours", "~2 days"). */