feat: board post + channel API routes

Implements Phase 2a of board classifieds redesign:

- GET/POST /api/board/posts (list with tag/author filters, create)
- PATCH/DELETE /api/board/posts/:id (author-only)
- GET /api/board/channels (member)
- POST /api/admin/board-channels (admin)
- PATCH/DELETE /api/admin/board-channels/:id (admin)

Adds board_post_created activity type.

server/api/admin/board-channels.post.js

@@ -0,0 +1,29 @@
+import BoardChannel from '../../models/boardChannel.js'
+import { requireAdmin } from '../../utils/auth.js'
+import { validateBody } from '../../utils/validateBody.js'
+import { boardChannelCreateSchema } from '../../utils/schemas.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+
+  const body = await validateBody(event, boardChannelCreateSchema)
+
+  try {
+    const channel = await BoardChannel.create({
+      name: body.name,
+      slackChannelId: body.slackChannelId,
+      tagSlugs: body.tagSlugs || []
+    })
+
+    setResponseStatus(event, 201)
+    return { channel: channel.toObject() }
+  } catch (err) {
+    if (err.code === 11000) {
+      throw createError({
+        statusCode: 409,
+        statusMessage: 'A channel with that Slack channel ID already exists'
+      })
+    }
+    throw err
+  }
+})