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/app/models.py

@@ -265,6 +265,25 @@ class ApiQuotaUsage(Base):
     units_used: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
 
 
+class QuotaEvent(Base):
+    """Per-spend audit log for YouTube API quota attribution. `user_id` is the end-user
+    whose action caused the spend; NULL means background/system work (scheduler backfill,
+    enrichment, resync). The aggregate budget still lives in ApiQuotaUsage; this is detail."""
+
+    __tablename__ = "quota_events"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    # SET NULL on user delete: keep the audit trail (it just becomes system-attributed).
+    user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="SET NULL"), index=True
+    )
+    action: Mapped[str] = mapped_column(String(40), index=True)
+    units: Mapped[int] = mapped_column(Integer)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), index=True
+    )
+
+
 class AppState(Base):
     """Single-row global app state (admin-controlled)."""