feat:實作 Coach Portal — 教練後台課程管理

後端：
- Migration：diving_offers 新增 provider_id（nullable FK）
- Migration：users.role ENUM 加入 provider 值
- Migration：diving_offers.spot 改為 nullable
- AuthController：registerProvider business_name 改為選填
- AuthController：updateProviderProfile 補上 certifications / dive_sites / services / facilities / website / social_media
- ProviderOfferController：教練課程 CRUD（index/show/store/update/destroy），實作 provider_id 所有權不變式（404 → 403 兩步驟）

前端（frontend/）：
- coachAuth store、coachAxios（獨立於 member auth）
- /coach/* 路由群組 + beforeEach guard
- CoachNavBar、CoachLayout（教練頁隱藏會員 NavBar）
- LoginView、RegisterView、DashboardView（表格 + 刪除確認）
- OfferFormView（新增/編輯共用）、ProfileView

OpenSpec：
- openspec/config.yaml 補入專案 context 與 rules
- 新增 specs：coach-offers-api / coach-portal-ui / provider-auth
- 更新 spec：diving-offers-api 加入 provider_id
- 歸檔：openspec/changes/archive/2026-05-10-coach-portal

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

database/migrations/2026_05_09_184151_add_provider_id_to_diving_offers_table.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->unsignedBigInteger('provider_id')->nullable()->after('id');
+            $table->foreign('provider_id')->references('id')->on('users')->onDelete('set null');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->dropForeign(['provider_id']);
+            $table->dropColumn('provider_id');
+        });
+    }
+};

database/migrations/2026_05_09_185046_add_provider_to_users_role_enum.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        DB::statement("ALTER TABLE users MODIFY COLUMN role ENUM('admin', 'coach', 'member', 'provider') NOT NULL DEFAULT 'member'");
+    }
+
+    public function down(): void
+    {
+        DB::statement("ALTER TABLE users MODIFY COLUMN role ENUM('admin', 'coach', 'member') NOT NULL DEFAULT 'member'");
+    }
+};

database/migrations/2026_05_09_185440_make_spot_nullable_in_diving_offers.php

@@ -0,0 +1,25 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->string('spot')->nullable()->change();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('diving_offers', function (Blueprint $table) {
+            $table->string('spot')->nullable(false)->change();
+        });
+    }
+};