Seating configurations: open or close venue sections (and half-sections) per resource

A venue today has one fixed seating layout: every seat it owns is always on sale. This adds seating configurations, so a venue can be opened in different ways (full house, reduced capacity, choir on stage) and a resource can run under a chosen one, with no seats duplicated.

Model:

• A new content entity yoyaku_seat_configuration (venue-scoped) stores only what it closes: closed_sections and closed_subsections. The full-house configuration stores nothing.
• Seats gain an optional integer subsection tag, so a configuration can close half of a section (a slice), not just whole sections.
• A seat is open when its section is not closed and its subsection, if any, is not closed.
• The venue gains an optional default_configuration. A resource keeps its venue field and gains an optional configuration that overrides the default. The active configuration for a slot is the resource configuration when set, otherwise the venue default, otherwise the whole venue (today behavior).

Behavior:

• The graphical seat map and seat availability show and offer only the seats open in the active configuration.
• Booking a closed seat is forbidden at the engine level: the SeatBookable constraint rejects it under the slot lock, so no booking path can take a closed seat.
• The configuration pickers are scoped to the relevant venue (an AJAX-dependent select on the resource form, a selection handler for the venue default).

The change is additive: no data migration, and venues or resources with no configuration behave exactly as today. Price grade stays on the seat for now.

Follow-ups (separate issues): round-tripping the new seat field and configurations in the import and export submodule, a graphical configuration editor, and per-configuration price-grade overrides.