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>

frontend/src/router/index.js

@@ -1,7 +1,9 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import { useAuthStore } from '../stores/auth'
+import { useCoachAuthStore } from '../stores/coachAuth'
 
 const routes = [
+  // Member
   { path: '/',               component: () => import('../views/HomeView.vue') },
   { path: '/courses',        component: () => import('../views/CoursesView.vue') },
   { path: '/courses/:id',    component: () => import('../views/CourseDetailView.vue') },
@@ -9,6 +11,22 @@ const routes = [
   { path: '/register',       component: () => import('../views/RegisterView.vue') },
   { path: '/auth/callback',  component: () => import('../views/AuthCallbackView.vue') },
   { path: '/profile',        component: () => import('../views/ProfileView.vue'), meta: { requiresAuth: true } },
+
+  // Coach (public)
+  { path: '/coach/login',    component: () => import('../views/coach/LoginView.vue') },
+  { path: '/coach/register', component: () => import('../views/coach/RegisterView.vue') },
+  // Coach (protected) — wrapped in CoachLayout
+  {
+    path: '/coach',
+    component: () => import('../layouts/CoachLayout.vue'),
+    meta: { requiresCoach: true },
+    children: [
+      { path: 'dashboard',       component: () => import('../views/coach/DashboardView.vue') },
+      { path: 'offers/new',      component: () => import('../views/coach/OfferFormView.vue') },
+      { path: 'offers/:id/edit', component: () => import('../views/coach/OfferFormView.vue') },
+      { path: 'profile',         component: () => import('../views/coach/ProfileView.vue') },
+    ],
+  },
 ]
 
 const router = createRouter({
@@ -17,10 +35,15 @@ const router = createRouter({
 })
 
 router.beforeEach((to) => {
-  const auth = useAuthStore()
+  const auth      = useAuthStore()
+  const coachAuth = useCoachAuthStore()
+
   if (to.meta.requiresAuth && !auth.isLoggedIn) {
     return { path: '/login' }
   }
+  if (to.meta.requiresCoach && !coachAuth.isLoggedIn) {
+    return { path: '/coach/login' }
+  }
 })
 
 export default router