Adds paliad.users.forum_pref so /tools/fristenrechner can pre-narrow
the proceeding picker to the user's typical inbox channel without
re-asking on every visit. The new column threads through the User
model, the userColumns SELECT, and UpdateProfileInput so the existing
PATCH /api/me handler accepts it without a new endpoint.
Allowed values mirror the channel chips m named in t-paliad-157:
- cms → UPC
- bea → national-DE
- posteingang → national-DE (slower channel, same forums)
NULL means "no preference, picker shows everything"; URL ?inbox=
overrides per-visit (frontend lands in the next commit). The CHECK
constraint enforces the 3-value enum at the DB layer; isValidForumPref
mirrors it in the service so callers see a typed error instead of a
raw pq violation. Empty string in the PATCH body clears the
preference, consistent with the EscalationContactID convention.
Migration 064 applied to the live Supabase pool; tracker bumped to
v64 so the boot-time runner skips re-applying.
Refs m/paliad#15.
