Allow characters to have multiple cast members assigned to them (under studies)

[Tim020]

Be good to be able to assign multiple cast members to a character, for example in an understudy capacity, and then at live show time allow selection of who is playing the character for that show and keep track of this.

[Tim020]

Design & Planning Complete

This issue has been scoped and broken into three sequential implementation issues, each independently implementable in its own session:

#	Issue	Depends on
1	#1112 — Add forward-planned session scheduling	—
2	#1113 — Add per-character eligible cast pool and per-session casting matrix	#1112
3	#1114 — Add session-aware mic allocation re-run from casting diff	#1113

Design decisions

The prototype explored several variants. Tim selected:

• Characters/casting model — Option C: A character × session matrix. Each character has an ordered eligible cast pool (principal + ranked understudies). Per-session casting is managed in a dedicated new Casting page — a grid where each cell can be changed from principal to a cover.

• Pre-show casting moment: Absorbed into the matrix via session scheduling. Sessions are now scheduled ahead (date/time/type/revision) and each becomes a column in the casting grid. No separate "pick tonight's cast" step needed.

• Mic re-assignment — Option C: A reviewable change-list showing per-slot diffs with per-row apply/skip checkboxes, a live-updating preview timeline, and a final modal to choose between "preserve manual overrides" and "wipe and regenerate from scratch".

Key constraints

• Once a session's start_date_time is set, its casting is locked (409 on any further casting changes)
• Casting is not shown in the live show view
• The existing Character.played_by field is kept for backward compatibility and stays in sync with the principal (priority-0 eligible cast member)
• session_type (Preview/Press Night/etc.) is a new built-in categorisation, separate from the existing user-defined SessionTag system