Add identity_type and guardian relationship to members

- Add identity_type field (patient/parent) to distinguish 病友/家長
- Add guardian_member_id for parent-child relationships
- Add guardian() and dependents() relationships
- Add isPatient(), isParent() helper methods
- Add identity_type_label accessor

Data updated:
- 47 members set as patient, 4 as parent
- 25 members with approved disability, 26 without
- 張序 linked to guardian 張誠駿
- Payment amounts corrected based on 2024 accounting records

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database/migrations/2026_01_25_043205_add_identity_and_guardian_to_members_table.php

@@ -0,0 +1,36 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * Adds identity_type (病友/家長) and guardian relationship fields.
+     */
+    public function up(): void
+    {
+        Schema::table('members', function (Blueprint $table) {
+            // 身份類型：病友 or 家長/父母
+            $table->string('identity_type')->nullable()->after('membership_type');
+
+            // 監護人關係（未成年人指向其父母的 member_id）
+            $table->foreignId('guardian_member_id')->nullable()->after('identity_type')
+                ->constrained('members')->nullOnDelete();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('members', function (Blueprint $table) {
+            $table->dropForeign(['guardian_member_id']);
+            $table->dropColumn(['identity_type', 'guardian_member_id']);
+        });
+    }
+};