perf(db): index transactions for the daily synced-email query

SendDailyBankTransactionsSyncedEmailJob filters transactions by
(user_id, source, created_at > ?). The only user_id-leading index is
idx_transactions_budget_lookup (user_id, transaction_date, category_id),
so MySQL narrows to the user's rows then scans them all to filter on
source + created_at — the slow query reported as PHP-LARAVEL-3X. Add a
(user_id, source, created_at) composite index matching the two
equalities and the range.

Fixes PHP-LARAVEL-3X

database/migrations/2026_07_02_133321_add_user_source_created_at_index_to_transactions_table.php

@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Support the daily "transactions synced" email query, which filters
+     * transactions by (user_id = ?, source = ?, created_at > ?). The existing
+     * indexes only lead with user_id via transaction_date, so that query scans
+     * every one of the user's rows to filter on source + created_at
+     * (PHP-LARAVEL-3X). This composite index matches the two equalities and the
+     * range directly.
+     */
+    public function up(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->index(['user_id', 'source', 'created_at'], 'idx_transactions_user_source_created');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->dropIndex('idx_transactions_user_source_created');
+        });
+    }
+};

tests/Feature/OpenBanking/DailyEmailTransactionsIndexTest.php

@@ -0,0 +1,14 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+
+it('has a composite index supporting the daily synced-email transaction query', function () {
+    $indexes = collect(Schema::getIndexes('transactions'));
+
+    $match = $indexes->first(fn (array $index): bool => $index['columns'] === ['user_id', 'source', 'created_at']
+    );
+
+    expect($match)->not->toBeNull(
+        'Expected a (user_id, source, created_at) index on transactions for the daily email query.'
+    );
+});