refactor(channelserver): eliminate *sql.Tx from repository interfaces

Hide transaction management inside repository implementations so
interfaces only expose domain types, enabling clean mocking and
decoupling handlers from PostgreSQL internals.

- Replace BeginTx + UpdateEventQuestStartTime with batch
  UpdateEventQuestStartTimes that manages its own transaction
- Remove tx parameter from CreateApplication, add composite
  CreateApplicationWithMail for atomic scout+mail operations
- Remove SendMailTx from MailRepo (sole caller migrated)
- Remove database/sql import from repo_interfaces.go

server/channelserver/repo_mail.go

@@ -1,8 +1,6 @@
 package channelserver
 
 import (
-	"database/sql"
-
 	"github.com/jmoiron/sqlx"
 )
 
@@ -27,12 +25,6 @@ func (r *MailRepository) SendMail(senderID, recipientID uint32, subject, body st
 	return err
 }
 
-// SendMailTx inserts a new mail row within an existing transaction.
-func (r *MailRepository) SendMailTx(tx *sql.Tx, senderID, recipientID uint32, subject, body string, itemID, itemAmount uint16, isGuildInvite, isSystemMessage bool) error {
-	_, err := tx.Exec(mailInsertQuery, senderID, recipientID, subject, body, itemID, itemAmount, isGuildInvite, isSystemMessage)
-	return err
-}
-
 // GetListForCharacter loads all non-deleted mail for a character (max 32).
 func (r *MailRepository) GetListForCharacter(charID uint32) ([]Mail, error) {
 	rows, err := r.db.Queryx(`