chore: 清除廢棄的 DB 表與 Model

- 新增 migration 刪除 coach_profiles、coach_member、plans、subscriptions 四張廢棄表
- 刪除對應的空殼 Model：CoachProfile、Plan、Subscription

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

database/migrations/2026_05_25_083147_drop_unused_tables.php

@@ -0,0 +1,20 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::dropIfExists('subscriptions');
+        Schema::dropIfExists('plans');
+        Schema::dropIfExists('coach_member');
+        Schema::dropIfExists('coach_profiles');
+    }
+
+    public function down(): void
+    {
+        // 這些表已確認廢棄，不提供還原
+    }
+};