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>

migrations/Version20260628165710.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260628165710 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return '';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('CREATE INDEX idx_wallet_user_type ON wallet_transactions (user_id, type)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX idx_wallet_user_type ON wallet_transactions');
+    }
+}