feat(ST-imalmyr7): POST /api/user-questions/[id]/answer — worker-antwoord endpoint

- Bearer-auth via authenticateApiRequest (token_hash SHA-256)
- Zod-validatie van body.answer (max 8000 chars)
- Guard: 404 bij onbekend id, 409 als al beantwoord
- Slaat answer op en zet status naar answered
- pg_notify scrum4me_changes met user_question entity-event

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

app/api/user-questions/[id]/answer/route.ts

@@ -0,0 +1,62 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { z } from 'zod'
+
+import { authenticateApiRequest } from '@/lib/api-auth'
+import { prisma } from '@/lib/prisma'
+
+interface RouteContext {
+  params: Promise<{ id: string }>
+}
+
+const bodySchema = z.object({
+  answer: z.string().min(1).max(8000),
+})
+
+export async function POST(request: NextRequest, ctx: RouteContext) {
+  const auth = await authenticateApiRequest(request)
+  if ('error' in auth) {
+    return NextResponse.json({ error: auth.error }, { status: auth.status })
+  }
+
+  const { id } = await ctx.params
+
+  let body: unknown
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'Malformed JSON' }, { status: 400 })
+  }
+
+  const parsed = bodySchema.safeParse(body)
+  if (!parsed.success) {
+    return NextResponse.json({ error: 'Ongeldige invoer', details: parsed.error.flatten() }, { status: 422 })
+  }
+
+  const uq = await prisma.userQuestion.findFirst({
+    where: { id },
+    select: { id: true, idea_id: true, status: true },
+  })
+  if (!uq) {
+    return NextResponse.json({ error: 'UserQuestion niet gevonden' }, { status: 404 })
+  }
+  if (uq.status !== 'pending') {
+    return NextResponse.json({ error: 'Vraag is al beantwoord' }, { status: 409 })
+  }
+
+  await prisma.userQuestion.update({
+    where: { id },
+    data: { answer: parsed.data.answer, status: 'answered' },
+  })
+
+  await prisma.$executeRaw`
+    SELECT pg_notify('scrum4me_changes', ${JSON.stringify({
+      op:       'U',
+      entity:   'user_question',
+      id,
+      idea_id:  uq.idea_id,
+      status:   'answered',
+    })}::text)
+  `
+
+  return NextResponse.json({ ok: true })
+}