feat(mail): keep mail a player deletes, marked deleted, for staff

Deleting mail in game now sets mail.deleted_at (migrations mysql 95,
sqlite 78) instead of removing the row. Every read for the game skips
deleted mail: the mailbox, a single mail (claim, read, delete), the
unread count, the economy scan of waiting attachments and the UGC
lookup of mailed models. Deleting a character still removes its mail.

Check: delete a mail in game; it leaves the mailbox, the unread count
drops, and the row is still in the mail table with deleted_at set.
Parity test: ParitySeeded.Mail (DeleteMail).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/dlu/sqlite/78_mail_deleted_at.sql

@@ -0,0 +1,2 @@
+/* mail.deleted_at: when the player deleted the mail in game. See the MySQL migration. */
+ALTER TABLE mail ADD COLUMN deleted_at BIGINT NOT NULL DEFAULT 0;