mockGuildRepoForMail and mockGuildRepoOps each implemented different
subsets of the 68-method GuildRepo interface. Adding any new method
required updating both mocks. Merged into a single mockGuildRepo with
configurable struct fields for error injection and no-op defaults for
the rest.
Introduce MailService as a convenience layer between handlers/services
and MailRepo. Provides Send, SendSystem, SendGuildInvite, and
BroadcastToGuild methods that encapsulate the boolean flag combinations.
GuildService now depends on MailService instead of MailRepo directly,
simplifying its mail-sending calls from verbose SendMail(..., false, true)
to clean SendSystem(recipientID, subject, body).
Guild mail broadcast logic moved from handleMsgMhfSendMail into
MailService.BroadcastToGuild.
