diff --git a/Dockerfile b/Dockerfile
index 9cd94a3..afe2e67 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -13,6 +13,7 @@ RUN npm ci
 COPY tsconfig.json vitest.config.ts ./
 COPY server ./server
 RUN npm run build:server && npm test
+RUN npm prune --omit=dev
 
 # ---- runtime stage ----
 FROM node:22-slim
diff --git a/server/src/routes/authRoutes.ts b/server/src/routes/authRoutes.ts
index 2ba047e..ca70abb 100644
--- a/server/src/routes/authRoutes.ts
+++ b/server/src/routes/authRoutes.ts
@@ -80,18 +80,25 @@ export async function registerAuthRoutes(app: FastifyInstance): Promise<void> {
     const { username, password } = (request.body ?? {}) as { username?: string; password?: string }
     const invalid = validateCredentials(username, password)
     if (invalid) return reply.code(400).send({ error: 'invalid_input', detail: invalid })
-    let user
-    try {
-      user = createUser(request.server.db, username as string, await hashPassword(password as string), true)
-    } catch (err: any) {
-      if (String(err.message).includes('UNIQUE constraint failed')) {
-        return reply.code(403).send({ error: 'setup_already_done' })
-      }
-      throw err
-    }
-    request.server.db.prepare('INSERT INTO settings (user_id) VALUES (?)').run(user.id)
-    const token = createSession(request.server.db, user.id)
-    return reply.setCookie(COOKIE_NAME, token, cookieOpts()).code(200).send({ user: toPublicUser(user) })
+    const passwordHash = await hashPassword(password as string)
+
+    const insertSetup = request.server.db.transaction((): 'taken' | { user: UserRow } => {
+      const current = (
+        request.server.db.prepare('SELECT COUNT(*) AS n FROM users').get() as { n: number }
+      ).n
+      if (current > 0) return 'taken'
+      const user = createUser(request.server.db, username as string, passwordHash, true)
+      request.server.db.prepare('INSERT INTO settings (user_id) VALUES (?)').run(user.id)
+      return { user }
+    })
+    const result = insertSetup.immediate()
+    if (result === 'taken') return reply.code(403).send({ error: 'setup_already_done' })
+
+    const token = createSession(request.server.db, result.user.id)
+    return reply
+      .setCookie(COOKIE_NAME, token, cookieOpts())
+      .code(200)
+      .send({ user: toPublicUser(result.user) })
   })
 
   app.post('/api/login', async (request, reply) => {