feat: #3612723 yoyaku_ticket_placement: show the seat on placed-event tickets

Bridges yoyaku_placement and yoyaku_ticket: a ticket for a placed booking is named with the place it admits to (its section and seat, for example "Parterre - L15"), shown on the ticket's Place line.

New submodule yoyaku_ticket_placement:

• TicketPlacementHooks implements the hook_yoyaku_ticket_units_alter seam: for each confirmed booking that occupies a place, it rebuilds the (immutable) ticket unit with the place in its place meta, which the ticket template already prints. The QR still names the same booking.
• A placed resource makes one booking per seat, so there is no granularity choice: one ticket per booking is one ticket per seat. Unplaced bookings pass through untouched.
• Only EntityTypeManager is injected (autowired #[Hook]); yoyaku_ticket stays unaware of placement and placement never depends on ticketing.

Also documents the seam in a new yoyaku_ticket.api.php, extends the Tickets doc Placement section, and adds the French translation. Covered by TicketPlacementTest (placed tickets carry their place; unplaced do not).