refactor(tower): extract tower logic into TowerService

The tower repo had business logic beyond simple CRUD: AddGem used a
fetch-transform-save pattern, progress capping was inline in the
handler, and RP donation orchestrated multiple repo calls with
conditional page advancement. Move these into a new TowerService
following the established service layer pattern.

server/channelserver/repo_tower.go

@@ -3,8 +3,6 @@ package channelserver
 import (
 	"fmt"
 
-	"erupe-ce/common/stringsupport"
-
 	"github.com/jmoiron/sqlx"
 )
 
@@ -78,16 +76,6 @@ func (r *TowerRepository) UpdateGems(charID uint32, gems string) error {
 	return err
 }
 
-// AddGem adds quantity to a specific gem index.
-func (r *TowerRepository) AddGem(charID uint32, gemIndex int, quantity int) error {
-	gems, err := r.GetGems(charID)
-	if err != nil {
-		return err
-	}
-	newGems := stringsupport.CSVSetIndex(gems, gemIndex, stringsupport.CSVGetIndex(gems, gemIndex)+quantity)
-	return r.UpdateGems(charID, newGems)
-}
-
 // TenrouiraiProgressData holds the guild's tenrouirai (sky corridor) progress.
 type TenrouiraiProgressData struct {
 	Page     uint8