feat(m5d): demand-driven deep backfill + per-user ETA

Per-user opt-in to full-history (deep) backfill so a new user's unique
channels no longer trigger a big shared-quota burst.

- migration 0007: Subscription.deep_requested (default false); seed admins'
  existing subscriptions to preserve today's global-backfill behaviour
- run_deep_backfill is now demand-driven: only channels at least one user has
  requested are deep-backfilled; recent backfill stays unconditional (cheap)
- estimate_deep_backfill ETA helper (quota-bound) surfaced in /api/sync/my-status
- POST /api/sync/deep-all to opt all my channels in; PATCH channels accepts
  deep_requested
- UI: per-channel Full history toggle, Backfill everything action, deep
  progress + ETA in Channels header and Settings - Sync

backend/app/models.py

@@ -112,6 +112,12 @@ class Subscription(Base):
     yt_subscription_id: Mapped[str | None] = mapped_column(String(128))
     priority: Mapped[int] = mapped_column(Integer, default=0, server_default="0")
     hidden: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
+    # Per-user opt-in to full-history (deep) backfill of this channel. Default off so a
+    # new user's unique channels don't trigger a big shared-quota burst; the deep
+    # scheduler only picks up channels at least one user has requested.
+    deep_requested: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false"
+    )
     subscribed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()