Soft-delete users instead of hard-deleting, add last login tracking

Fixes the 500 on admin user deletion: deleting a user with an
email_log row (i.e. basically anyone who received any email) hit an
unhandled ForeignKeyConstraintViolationException, since email_log,
reset_password_request and player all have non-nullable FKs to user
with no cascade at the DB level.

Instead of cascading the delete (which would be fine for email_log
and reset_password_request but risky for player - removing a player
row could corrupt other real players' session state), admin delete
now sets a deletedAt timestamp instead of removing the row:
- UserChecker blocks login for deleted users (checkPreAuth).
- EmailLoggerListener rejects the message before send for deleted
  recipients (checked at actual send time, not at queue time).
- Added a last_login_at column + a LoginSuccessEvent listener to
  populate it, and surfaced both last login and status in the admin
  users list.

Added `app:users:purge-deleted`, a command intended to run on a
schedule that permanently removes users who were soft-deleted more
than 3 months ago and never played a game (join to Player via a
NOT EXISTS subquery). For that eventual hard-delete to actually
succeed, email_log and reset_password_request now cascade-delete at
the DB level (migration drops+recreates both FK constraints with ON
DELETE CASCADE) - player intentionally still isn't cascaded, so a
user with game history can never be purged this way even by mistake.

src/Tech/Service/UserChecker.php

@@ -16,6 +16,10 @@ class UserChecker implements UserCheckerInterface
             return;
         }
 
+        if ($user->isDeleted()) {
+            throw new CustomUserMessageAuthenticationException('This account no longer exists.');
+        }
+
         if (!$user->isVerified()) {
             throw new CustomUserMessageAuthenticationException('Your email address is not verified.', ['%resend_link%' => '/verify/resend']);
         }