feat:實作 API 效能優化 — Redis 快取、分頁、DB 索引

- 引入 Redis（predis）快取層：Admin Stats（5分鐘）、課程列表（3分鐘，tag-based 失效）、評價分布（10分鐘）
- ReviewController::publicList 改為 paginate + eager load votes，消除 N+1
- AdminReviewController::index 加入分頁（預設 20，最大 100）
- 新增 notifications / diving_offers 效能索引 migration
- 新增 docker-compose.override.yml 本機開發 port mapping 機制（不進 git）

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

database/migrations/2026_05_18_172221_add_performance_indexes.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
+{
+    public function up(): void
+    {
+        Schema::table('notifications', function (Blueprint $table) {
+            $table->index(['notifiable_type', 'notifiable_id', 'read_at'], 'notifications_notifiable_read_at_index');
+        });
+
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->index('provider_id', 'diving_offers_provider_id_index');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('notifications', function (Blueprint $table) {
+            $table->dropIndex('notifications_notifiable_read_at_index');
+        });
+
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->dropIndex('diving_offers_provider_id_index');
+        });
+    }
+};