notification_preferences (048) had FKs to BOTH space_members and spaces.
PostgREST read that as a junction, so embedding spaces into space_members
(the auth layout's `select('*, spaces(*)')`) became ambiguous and errored
-- member resolved to null and every logged-in user was bounced to /signup.
Drop the redundant space_id -> spaces FK (space is reachable via
member_id -> space_members; space deletion still cascades through it). With
one FK left the table is no longer a junction and the embed is unambiguous.
Verified against local PostgREST: the embed returns rows again.
(schema.sql also carries the in-progress notifications.read_at column from
uncommitted inbox work; harmless -- prod applies migrations, not schema.sql.)
