Model the venue as a first-class place: sections, seat grades and numbered seats

Yoyaku's single capacity axis (the per-slot category/tier) conflated the physical zone with the price class. This issue makes the venue and its layout first-class, so a hall's seats are defined once and reused across every event that plays there.

Model (delivered by MR !38):

• Venue (yoyaku_venue): the physical place, shared across events. A resource runs in a venue; a slot inherits it.
• Section (yoyaku_section): a named venue area (Parterre, Balcon, Galerie), stable across events, translatable label.
• Seat grade (yoyaku_seat_grade): the venue's quality-of-viewing band (Optima, Cat. 1 to Cat. 9), operator-named and translatable, with a colour. Reflects seat quality and cuts across sections.
• Seat (yoyaku_seat, from #3611637): belongs to the venue, sits in a section and a grade, and carries row and position geometry.
• Category (yoyaku_category): groups one or more grades and carries the price, so an event prices seats by viewing quality; a constraint keeps a booked seat's grade within its category's grades.
• Party-adjacency assignment: the best free seats for a party (a contiguous run when possible), held atomically under the slot lock.

Remaining (follow-ups):

• General admission and mixed venues: a section mode (seated / general admission), GA standing pools, per-slot section enable/disable, pool availability.
• Per-event pricing (price per grade and rate) and the price-aware seat picker.
• Admin UI to manage venues, sections, grades and seats, plus a bulk seat generator.
• Graphical seat map; tickets (#3611638); check-in (#3611639).