feat(stats): per-user API quota attribution + admin usage page

Track who burned how much YouTube API quota. A QuotaEvent audit log (migration
0009) records every spend with the triggering user (NULL = background/system) and
an action label, set via a request/job-scoped contextvar (quota.attribute) so no
call signatures change. User-initiated work (sync subscriptions, unsubscribe,
opt-in recent backfill, manual enrich) attributes to the user; scheduler work to
System, split by action.

- backend: QuotaEvent model + migration 0009; quota.attribute() contextvar;
  record_usage logs events; entry points wrapped (routes/sync, routes/channels,
  scheduler); GET /api/quota/my-usage + GET /api/quota/admin
- frontend: admin-only Stats page (header nav, page=stats) with daily bars +
  per-user breakdown by action and range picker; 'Your API usage' in Settings ->
  Sync for every user

Verified: attribution + endpoints compute correctly; events are per-user vs System.

backend/alembic/versions/0009_quota_events.py

@@ -0,0 +1,46 @@
+"""quota_events: per-spend audit log for per-user quota attribution
+
+Revision ID: 0009_quota_events
+Revises: 0008_invites
+Create Date: 2026-06-12
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0009_quota_events"
+down_revision: Union[str, None] = "0008_invites"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "quota_events",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "user_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+        sa.Column("action", sa.String(length=40), nullable=False),
+        sa.Column("units", sa.Integer(), nullable=False),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+    )
+    op.create_index("ix_quota_events_user_id", "quota_events", ["user_id"])
+    op.create_index("ix_quota_events_action", "quota_events", ["action"])
+    op.create_index("ix_quota_events_created_at", "quota_events", ["created_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_quota_events_created_at", table_name="quota_events")
+    op.drop_index("ix_quota_events_action", table_name="quota_events")
+    op.drop_index("ix_quota_events_user_id", table_name="quota_events")
+    op.drop_table("quota_events")