feat(enakgame): weekly leaderboard

GET /customer/enakgame/leaderboard ranks the customers of an organization by
the EnakCoin they won from games in one week, Monday to Sunday in
Asia/Jakarta. Only completed, unflagged sessions count, by the week they
started; entry cost is not subtracted. Equal totals rank by who reached
theirs first, so no two customers share a rank.

Optional date (any day of the week, this week by default), game_id and limit
(10, at most 100). The response holds the top ranks and, under me, the
customer asking wherever they rank. Other customers are shown by a masked
name: the first word whole, the others by their first letter; a word with a
digit keeps only its first character.

The EnakGame web game shows it: integration-enakgame.md §10 (checklist now
§11) and RFC §11. The DB test needs TEST_DATABASE_URL.

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

internal/router/router.go

@@ -197,6 +197,7 @@ func (r *Router) addAppRoutes(rg *gin.Engine) {
 				enakGame.GET("/sessions", r.enakGameCustomerHandler.ListSessions)
 				enakGame.GET("/sessions/:id", r.enakGameCustomerHandler.GetSession)
 				enakGame.POST("/sessions/:id/complete", r.enakGameCustomerHandler.CompleteSession)
+				enakGame.GET("/leaderboard", r.enakGameCustomerHandler.Leaderboard)
 			}
 
 			// Vouchers: what EnakPoint is spent on, whatever it came from (docs/rfc-enakgame.md