Add a main-JTP designation to comisiones

A comisión has NO main-JTP concept today: there is no column on comisiones naming a responsible JTP. The student->JTP link exists (studentAssignments carries one JTP slot per roster row) but comisión->main-JTP does not, so the app cannot name the person who owns a comisión. Blocks the client-requested comisión close/reopen feature (Endodoncia, classes end 18/12 but students keep uploading after): the ask is that the MAIN JTP of each comisión closes it, and there is currently nobody to grant that to. Scope to decide before implementing: - column on comisiones (e.g. mainJtpUserId, nullable FK -> users) vs a flag on the existing assignment rows. A column is the smaller change and matches the one-per-comisión semantics. - who sets it: admin-side per Elazar, not self-service. - backfill: not automatic. Not every comisión has an obvious candidate, and the JTP with the most graded prácticas is a guess, not a designation. Needs a per-comisión human pass, and some comisiones may need someone other than the current JTP if that JTP has never logged in. - co-JTP comisiones cannot be represented by the existing single-slot studentAssignments model; a main-JTP column does not fix that and should not be expected to. Related: SOP-MARS-39 / the entregaCierreAt movable-cut-off design (close/reopen must be a movable date presented as Cerrar/Reabrir, never a terminal state).