Add a generic, durable squad->manager assignment (migration 0023
squad_managers) so a manager persists across squad save, reload, and
server restart, backed by authoritative Core owned_cards.
- squad_managers(squad_id PK, owned_card_id, updated_at) with ON DELETE
CASCADE on both FKs: quick-selling the manager auto-clears the
assignment (no resurrection); one manager per squad (no duplicates).
- club::{get,set,clear}_squad_manager validate club ownership of both the
squad and the card, and re-check ownership on read (defends against a
stale row left by a market transfer).
- GET/PUT /club/manager routes expose the assignment; FIFA wire meaning
stays in the adapter.
Tests: persistence across reload+restart (headline), reassignment
replace/no-duplicate, clear/no-resurrection, cascade on quick-sell,
foreign-card rejection.
funman300