feat(ST-703): auth and Prisma client singleton

- src/prisma.ts: PrismaClient via PrismaPg adapter and pg.Pool,
  same pattern as Scrum4Me's lib/prisma.ts
- src/auth.ts: getAuth() resolves SCRUM4ME_TOKEN once, caches
  { userId, username, isDemo }. requireWriteAccess() throws
  PermissionDeniedError for demo tokens — write tools call this
  before any DB mutation

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/auth.ts

@@ -0,0 +1,51 @@
+import { createHash } from 'crypto'
+import { prisma } from './prisma.js'
+
+export type AuthContext = {
+  userId: string
+  username: string
+  isDemo: boolean
+}
+
+let cached: AuthContext | null = null
+
+export async function getAuth(): Promise<AuthContext> {
+  if (cached) return cached
+
+  const token = process.env.SCRUM4ME_TOKEN
+  if (!token) {
+    throw new Error('SCRUM4ME_TOKEN is not set — see .env.example')
+  }
+
+  const tokenHash = createHash('sha256').update(token).digest('hex')
+  const apiToken = await prisma.apiToken.findUnique({
+    where: { token_hash: tokenHash },
+    include: { user: true },
+  })
+
+  if (!apiToken || apiToken.revoked_at) {
+    throw new Error('SCRUM4ME_TOKEN is invalid or revoked')
+  }
+
+  cached = {
+    userId: apiToken.user_id,
+    username: apiToken.user.username,
+    isDemo: apiToken.user.is_demo,
+  }
+  return cached
+}
+
+export class PermissionDeniedError extends Error {
+  constructor(message = 'Demo accounts cannot perform write operations') {
+    super(message)
+    this.name = 'PermissionDeniedError'
+  }
+}
+
+export async function requireWriteAccess(): Promise<AuthContext> {
+  const auth = await getAuth()
+  if (auth.isDemo) {
+    throw new PermissionDeniedError()
+  }
+  return auth
+}

src/prisma.ts

@@ -0,0 +1,15 @@
+import { PrismaClient } from '@prisma/client'
+import { Pool } from 'pg'
+import { PrismaPg } from '@prisma/adapter-pg'
+
+function createClient(): PrismaClient {
+  const url = process.env.DATABASE_URL
+  if (!url) {
+    throw new Error('DATABASE_URL is not set — see .env.example')
+  }
+  const pool = new Pool({ connectionString: url })
+  const adapter = new PrismaPg(pool)
+  return new PrismaClient({ adapter, log: ['error'] })
+}
+
+export const prisma = createClient()