feat(bulletin): add group bulletin board (信息公示板)

- Add bulletin_posts and bulletin_reads PocketBase collections with migrations
- Add BulletinPost/BulletinRead types and priority levels (low/normal/high/urgent)
- Add bulletin API (CRUD, read tracking, realtime subscription)
- Add bulletin Pinia store with pinned/normal sections and expiration filtering
- Add BulletinPinned header component for pinned announcements
- Add BulletinBoard tab component with sectioned list layout
- Add BulletinPostCard with priority tags, unread dots, and hover actions
- Add CreateBulletinDialog with title, content, priority, pinned, expiresAt
- Integrate bulletin tab into GroupView with Bell icon and real-time updates

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

frontend/src/api/bulletins.ts

@@ -0,0 +1,113 @@
+import { pb } from './pocketbase'
+import type { BulletinPost, BulletinRead } from '@/types'
+
+export async function createBulletin(data: {
+  group: string
+  title: string
+  content: string
+  priority?: 'low' | 'normal' | 'high' | 'urgent'
+  pinned?: boolean
+  expiresAt?: string
+}): Promise<BulletinPost> {
+  const user = pb.authStore.model
+  if (!user) throw new Error('未登录')
+
+  const post = await pb.collection('bulletin_posts').create({
+    group: data.group,
+    creator: user.id,
+    title: data.title,
+    content: data.content,
+    priority: data.priority || 'normal',
+    pinned: data.pinned || false,
+    expiresAt: data.expiresAt || '',
+  })
+  return post as unknown as BulletinPost
+}
+
+export async function listBulletins(groupId: string): Promise<BulletinPost[]> {
+  const result = await pb.collection('bulletin_posts').getFullList({
+    filter: `group="${groupId}"`,
+    sort: '-pinned,-created',
+    expand: 'creator',
+    $autoCancel: false,
+  })
+  return result as unknown as BulletinPost[]
+}
+
+export async function getBulletin(postId: string): Promise<BulletinPost> {
+  const result = await pb.collection('bulletin_posts').getOne(postId, {
+    expand: 'creator',
+  })
+  return result as unknown as BulletinPost
+}
+
+export async function updateBulletin(
+  postId: string,
+  data: {
+    title?: string
+    content?: string
+    priority?: 'low' | 'normal' | 'high' | 'urgent'
+    pinned?: boolean
+    expiresAt?: string
+  }
+): Promise<BulletinPost> {
+  const payload: Record<string, any> = {}
+  if (data.title !== undefined) payload.title = data.title
+  if (data.content !== undefined) payload.content = data.content
+  if (data.priority !== undefined) payload.priority = data.priority
+  if (data.pinned !== undefined) payload.pinned = data.pinned
+  if (data.expiresAt !== undefined) payload.expiresAt = data.expiresAt
+
+  const result = await pb.collection('bulletin_posts').update(postId, payload)
+  return result as unknown as BulletinPost
+}
+
+export async function deleteBulletin(postId: string): Promise<void> {
+  await pb.collection('bulletin_posts').delete(postId)
+}
+
+// 已读追踪
+export async function markBulletinAsRead(postId: string): Promise<BulletinRead | null> {
+  const user = pb.authStore.model
+  if (!user) return null
+
+  // 检查是否已存在
+  const existing = await pb.collection('bulletin_reads').getList(1, 1, {
+    filter: `post="${postId}" && user="${user.id}"`,
+    $autoCancel: false,
+  })
+  if (existing.items.length > 0) return existing.items[0] as unknown as BulletinRead
+
+  const result = await pb.collection('bulletin_reads').create({
+    post: postId,
+    user: user.id,
+  })
+  return result as unknown as BulletinRead
+}
+
+export async function listMyReads(): Promise<BulletinRead[]> {
+  const user = pb.authStore.model
+  if (!user) return []
+
+  const result = await pb.collection('bulletin_reads').getFullList({
+    filter: `user="${user.id}"`,
+    $autoCancel: false,
+  })
+  return result as unknown as BulletinRead[]
+}
+
+// 实时订阅
+export async function subscribeBulletins(
+  groupId: string,
+  callback: (data: any) => void
+): Promise<() => void> {
+  await pb.collection('bulletin_posts').subscribe('*', (data) => {
+    if (data.record?.group === groupId) {
+      callback(data)
+    }
+  })
+
+  return () => {
+    pb.collection('bulletin_posts').unsubscribe('*')
+  }
+}