fix: add FK constraints on feature_code columns to entitlement_features

Add foreign key constraints from usage_alert_history.feature_code,
entitlement_boosts.feature_code, and entitlement_usage_records.feature_code
to entitlement_features.code to prevent orphaned records.

Uses cascadeOnUpdate (code renames propagate) and restrictOnDelete
(cannot delete a feature that has usage/alert/boost records).

Fixes #12

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

Migrations/2026_03_24_000000_add_feature_code_foreign_keys.php

@@ -0,0 +1,57 @@
+<?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 foreign key constraints on feature_code columns to prevent orphaned records.
+     *
+     * Fixes #12: feature_code in usage_alert_history not constrained to entitlement_features.
+     */
+    public function up(): void
+    {
+        Schema::table('entitlement_usage_alert_history', function (Blueprint $table) {
+            $table->foreign('feature_code', 'usage_alert_feature_code_fk')
+                ->references('code')
+                ->on('entitlement_features')
+                ->cascadeOnUpdate()
+                ->restrictOnDelete();
+        });
+
+        Schema::table('entitlement_boosts', function (Blueprint $table) {
+            $table->foreign('feature_code', 'boosts_feature_code_fk')
+                ->references('code')
+                ->on('entitlement_features')
+                ->cascadeOnUpdate()
+                ->restrictOnDelete();
+        });
+
+        Schema::table('entitlement_usage_records', function (Blueprint $table) {
+            $table->foreign('feature_code', 'usage_records_feature_code_fk')
+                ->references('code')
+                ->on('entitlement_features')
+                ->cascadeOnUpdate()
+                ->restrictOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('entitlement_usage_alert_history', function (Blueprint $table) {
+            $table->dropForeign('usage_alert_feature_code_fk');
+        });
+
+        Schema::table('entitlement_boosts', function (Blueprint $table) {
+            $table->dropForeign('boosts_feature_code_fk');
+        });
+
+        Schema::table('entitlement_usage_records', function (Blueprint $table) {
+            $table->dropForeign('usage_records_feature_code_fk');
+        });
+    }
+};