feat(squads): PromoteToSubCaptain / demote (#358)

[guisaliba]

Closes #358

Parent: #342

What changed

• Added PromoteToSubCaptain with handle() and demote() methods.
• Added SquadPolicy authorization for captains, sub-captains, and super-admins.
• Added transactional, locked role changes with append-only membership events.
• Added idempotent same-state no-ops.
• Added direct captain -> sub-captain and captain -> member transitions for off-platform election and succession outcomes.
• Added feature coverage for authorization, inactive memberships, self-actions, vacancies, and event details.

Event scope

The existing promote and demote event values remain unchanged. from_role and to_role identify each transition. AssignCaptain remains responsible for assigning a captain; this action records the downward role changes.

Verification

• vendor/bin/pest --compact app-modules/squads/tests/Feature/PromoteToSubCaptainTest.php
• vendor/bin/pest --compact app-modules/squads/tests
• make test
• make test-pint
• make test-rector
• make phpstan

All checks pass.