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>

tests/Smoke/InfraSmokeTest.php

@@ -31,4 +31,16 @@ class InfraSmokeTest extends ApiTestCase
         $this->assertTrue($body['success']);
         $this->assertSame($user->getUuid(), $body['data']['uuid']);
     }
+
+    public function testWalletUserTypeIndexExists(): void
+    {
+        $conn  = $this->em->getConnection();
+        $count = (int) $conn->fetchOne(
+            "SELECT COUNT(*) FROM information_schema.STATISTICS
+              WHERE TABLE_SCHEMA = DATABASE()
+                AND TABLE_NAME = 'wallet_transactions'
+                AND INDEX_NAME = 'idx_wallet_user_type'"
+        );
+        $this->assertGreaterThan(0, $count, 'composite index idx_wallet_user_type is missing');
+    }
 }