Rename yoyaku_seating to yoyaku_placement and add standing (non-seated) sections

Problem/Motivation

Once booking gains an individual mode (#3612598), a venue must lay out both seated sections (named seats) and standing sections (zones with a capacity and no named seats), for example a stadium with seated stands and a standing pit. yoyaku_seating models only named seats, and its name no longer fits once it also handles standing placement.

Proposed resolution

1. Rename yoyaku_seating to yoyaku_placement and its vocabulary: seat becomes place; grade generalizes to cover seated and standing pricing; venue and configuration stay.
2. Add standing sections: a section with a capacity and no place entities (a zone). A standing booking references its section; a seated booking references a named place.
3. Move seat and zone assignment behind the core placement seam from #3612598, so seated and standing share one path.
4. Rename the yoyaku_seating_io sibling to match.

Remaining tasks

• Rename module, entities, services, routes, JS payload.
• Standing sections model, availability, pricing by grade.
• Placement provider plugged into the core seam.
• Tests.

Phase 2 of the placement generalization. Supersedes #3612561. Depends on #3612598.