make permanent assigned desks have "available days"

[c0dewhacker]

Accommodate people working part time. Have the ability for a permanently assigned user to make their desk always available on certain days.

[c0dewhacker]

Implementation plan (minor)

• Builds directly on the existing AssetAvailabilityWindow + the ASSIGNED-desk branch in assertBookable (apps/api/src/lib/booking.ts). Add a recurring weekday availability rule (e.g. "always available Mon/Fri") that the assigned user manages from the asset panel, expanded into windows (or matched directly at booking time).
• Effort: small — extends existing availability-window machinery; no new authz.