perf: add composite index on user_workspace(workspace_id, role)

Queries filtering by workspace_id and role (e.g. "get all owners of
workspace X") required a full table scan on the user_workspace pivot
table. This adds a composite index to cover those lookups efficiently.

Fixes #11

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Migrations/2026_03_24_000000_add_workspace_role_composite_index.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Add composite index on user_workspace(workspace_id, role).
+     *
+     * Fixes #11: queries like "get all owners of workspace X" previously
+     * required a full table scan. This composite index covers lookups
+     * filtering by workspace_id and role together.
+     */
+    public function up(): void
+    {
+        Schema::table('user_workspace', function (Blueprint $table) {
+            $table->index(['workspace_id', 'role'], 'user_workspace_workspace_role_idx');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('user_workspace', function (Blueprint $table) {
+            $table->dropIndex('user_workspace_workspace_role_idx');
+        });
+    }
+};