diff --git a/server/auth.js b/server/auth.js
index 2596ce1..3d8739b 100644
--- a/server/auth.js
+++ b/server/auth.js
@@ -117,9 +117,38 @@ function seedAdmin(adminEmail) {
   return Number(existing.id);
 }
 
+/**
+ * Migriert die (globale) Single-User-Datenbasis auf Mehrbenutzer, indem alle
+ * Alt-Zeilen dem Admin zugeordnet werden. Atomar (eine Transaktion), idempotent
+ * (Guard über die documents.user_id-Spalte). Muss NACH seedAdmin laufen.
+ * @returns {boolean} true wenn migriert, false wenn bereits migriert (No-Op)
+ */
+function migrateToMultiUser(adminUserId) {
+  const cols = db.prepare("PRAGMA table_info('documents')").all().map(c => c.name);
+  if (cols.includes('user_id')) return false; // schon migriert
+  const tx = db.transaction(() => {
+    db.exec(`CREATE TABLE documents_v2 (
+      user_id    INTEGER NOT NULL,
+      key        TEXT NOT NULL,
+      value      TEXT NOT NULL,
+      updated_at TEXT NOT NULL,
+      PRIMARY KEY (user_id, key)
+    )`);
+    db.prepare('INSERT INTO documents_v2 (user_id, key, value, updated_at) SELECT ?, key, value, updated_at FROM documents')
+      .run(adminUserId);
+    db.exec('DROP TABLE documents');
+    db.exec('ALTER TABLE documents_v2 RENAME TO documents');
+    const hcols = db.prepare("PRAGMA table_info('history')").all().map(c => c.name);
+    if (!hcols.includes('user_id')) db.exec('ALTER TABLE history ADD COLUMN user_id INTEGER');
+    db.prepare('UPDATE history SET user_id = ? WHERE user_id IS NULL').run(adminUserId);
+  });
+  tx();
+  return true;
+}
+
 module.exports = {
   normalizeEmail, hashToken, createLoginToken, consumeLoginToken,
   createSession, validateSession, deleteSession, deleteUserSessions,
-  seedAdmin,
+  seedAdmin, migrateToMultiUser,
   TOKEN_TTL_MIN, SESSION_TTL_DAYS, SESSION_IDLE_HOURS,
 };
diff --git a/server/migrate.test.js b/server/migrate.test.js
new file mode 100644
index 0000000..ab860ef
--- /dev/null
+++ b/server/migrate.test.js
@@ -0,0 +1,39 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+
+process.env.DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), 'dp-migrate-'));
+const { db } = require('./db');
+const { migrateToMultiUser, seedAdmin } = require('./auth');
+
+test('Migration ordnet Alt-Daten dem Admin zu, setzt neues PK-Schema, ist idempotent', () => {
+  // Alt-Schema (Single-User) erzwingen — unabhängig von der aktuellen db.js-Definition
+  db.exec("DROP TABLE IF EXISTS documents; CREATE TABLE documents (key TEXT PRIMARY KEY, value TEXT NOT NULL, updated_at TEXT NOT NULL)");
+  db.exec("DROP TABLE IF EXISTS history; CREATE TABLE history (id INTEGER PRIMARY KEY AUTOINCREMENT, key TEXT NOT NULL, value TEXT NOT NULL, replaced_at TEXT NOT NULL)");
+  const now = new Date().toISOString();
+  db.prepare('INSERT INTO documents (key,value,updated_at) VALUES (?,?,?)').run('employees', '["Max"]', now);
+  db.prepare('INSERT INTO documents (key,value,updated_at) VALUES (?,?,?)').run('duties', '{}', now);
+  db.prepare('INSERT INTO history (key,value,replaced_at) VALUES (?,?,?)').run('employees', '[]', now);
+
+  const adminId = seedAdmin('admin@x.de');
+  assert.strictEqual(migrateToMultiUser(adminId), true);
+
+  const cols = db.prepare("PRAGMA table_info('documents')").all().map(c => c.name);
+  assert.ok(cols.includes('user_id'), 'documents hat user_id');
+
+  const rows = db.prepare('SELECT user_id, key FROM documents ORDER BY key').all();
+  assert.deepStrictEqual(rows, [
+    { user_id: adminId, key: 'duties' },
+    { user_id: adminId, key: 'employees' },
+  ]);
+  assert.strictEqual(db.prepare('SELECT user_id FROM history').get().user_id, adminId);
+
+  // PK ist (user_id,key): dieselbe Kombi doppelt schlägt fehl
+  assert.throws(() => db.prepare('INSERT INTO documents (user_id,key,value,updated_at) VALUES (?,?,?,?)')
+    .run(adminId, 'duties', '{}', now));
+
+  // Idempotent
+  assert.strictEqual(migrateToMultiUser(adminId), false);
+});