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/alembic/versions/0020_rename_quota_actions.py

@@ -0,0 +1,51 @@
+"""rename quota_events.action to the canonical <entity>_<action> taxonomy
+
+Revision ID: 0020_rename_quota_actions
+Revises: 0019_user_yt_channel_id
+Create Date: 2026-06-19
+
+The quota-attribution action keys were inconsistent (mixed verbs, ad-hoc names). Rename the
+stored historical values to the canonical scheme defined in app.quota.QuotaAction:
+``<entity>_<action>``, snake_case, explicit pull/push. Pure data migration on quota_events;
+fully reversible. (Scheduler job ids and progress phase labels are a separate namespace and
+are NOT touched.)
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0020_rename_quota_actions"
+down_revision: Union[str, None] = "0019_user_yt_channel_id"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+# old action value -> new canonical key (mirrors app.quota.QuotaAction).
+RENAMES: list[tuple[str, str]] = [
+    ("sync_subscriptions", "subscriptions_pull"),
+    ("subscription_resync", "subscriptions_resync"),
+    ("playlist_sync", "playlists_pull"),
+    ("playlist_push", "playlists_push"),
+    ("playlist_delete", "playlists_delete"),
+    ("backfill_recent", "videos_backfill_recent"),
+    ("backfill_deep", "videos_backfill_full"),
+    ("enrich", "videos_enrich"),
+    ("video_lookup", "videos_lookup"),
+    ("discovery", "channels_discover"),
+    ("subscribe", "channels_subscribe"),
+    ("unsubscribe", "channels_unsubscribe"),
+    ("maintenance", "maintenance_revalidate"),
+    ("api", "other"),
+]
+
+_SQL = sa.text("UPDATE quota_events SET action = :to WHERE action = :frm")
+
+
+def upgrade() -> None:
+    for frm, to in RENAMES:
+        op.execute(_SQL.bindparams(frm=frm, to=to))
+
+
+def downgrade() -> None:
+    for frm, to in RENAMES:
+        op.execute(_SQL.bindparams(frm=to, to=frm))