refactor(vouchers): move voucher routes out of /enakgame

Vouchers are what EnakPoint is redeemed for, wherever it came from, so they are
not part of EnakGame. Their only link to it is the budget attribution, which
does not change.

- Admin: /marketing/enakgame/vouchers... -> /marketing/vouchers...
- Customer: /customer/enakgame/vouchers -> /customer/vouchers,
  /customer/enakgame/vouchers/:id/redeem -> /customer/vouchers/:id/redeem,
  /customer/enakgame/redemptions -> /customer/vouchers/redemptions

Roles, handlers and logic stay the same. No client calls these endpoints yet.
RFC §7.4 and §11 updated.

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

internal/contract/enakgame_contract.go

@@ -8,7 +8,7 @@ type StartGameSessionRequest struct {
 	GameID uuid.UUID `json:"game_id" binding:"required"`
 }
 
-// RedeemVoucherRequest is POST /customer/enakgame/vouchers/:id/redeem (§7.4). The
+// RedeemVoucherRequest is POST /customer/vouchers/:id/redeem (§7.4). The
 // Idempotency-Key header is required.
 type RedeemVoucherRequest struct {
 	Pin string `json:"pin" binding:"required"`