The feed query was always scoped to the user's own non-hidden subscriptions
via an INNER JOIN on subscriptions. Add a scope param: scope=my (default)
keeps that behaviour; scope=all LEFT-joins the subscription instead, so every
video in the shared catalog shows while per-channel priority still resolves
for channels the user is subscribed to. Per-user watch state stays private via
the VideoState outer join in both modes. The priority sort is made null-safe
(coalesce to 0) since unsubscribed channels have no subscription row in all-mode.
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