As a program host I want to edit my program item, provided that it is editable

As a program host I want to edit my program item, provided that the program manager has marked it editable.

## Current state (2026-09-02)

* A program *offer* can be edited by its owner only while in state `new`: `ProgramOfferWorkflow.response_can_be_edited_by_owner` (`kompassi/program_v2/workflows/program_offer.py`). Once accepted, neither the offer nor the resulting program item can be edited by the host.
* The profile program view (`/profile/program`) shows hosted program items with a view link only; the edit link exists only for editable offers.
* There is no host-facing program item edit route in the frontend and no host-scoped update mutation (`updateProgram` is CBAC admin gated).

## Needed

* A per-program (or per-event) toggle for whether hosts may edit, and a definition of which fields are host-editable (schedule, dimensions etc. presumably not).
* Host-facing edit view + mutation that checks involvement (program host of this program) rather than admin CBAC.
* Decide how host edits relate to the accepted program offer / form response versioning (see also #728, #990).