perf(db): composite index on wallet_transactions(user_id, type) (M14)

Helps the per-type SUM balance query. M13 (service_items.section_id) and M15
(clinic_doctor_invitations.doctor_id) were false positives — both columns carry
a FK and are therefore auto-indexed by InnoDB; verified against the live schema.

Structural regression: InfraSmokeTest::testWalletUserTypeIndexExists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Settlement/Entity/WalletTransaction.php

@@ -11,6 +11,7 @@ use Symfony\Component\Uid\Uuid;
 #[ORM\Entity(repositoryClass: WalletTransactionRepository::class)]
 #[ORM\Table(name: 'wallet_transactions')]
 #[ORM\Index(columns: ['user_id', 'created_at'], name: 'idx_wallet_user_date')]
+#[ORM\Index(columns: ['user_id', 'type'], name: 'idx_wallet_user_type')]
 class WalletTransaction
 {
     public const TYPE_CREDIT = 'credit';