feat(backend): SQLite-Schicht mit WAL, Dokumenten und Historie

server/db.js

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+const path = require('path');
+const fs = require('fs');
+const Database = require('better-sqlite3');
+
+const DATA_DIR = process.env.DATA_DIR || path.join(__dirname, '..', 'data');
+fs.mkdirSync(DATA_DIR, { recursive: true });
+const DB_PATH = path.join(DATA_DIR, 'dienstplan.db');
+
+const db = new Database(DB_PATH);
+db.pragma('journal_mode = WAL');
+db.pragma('synchronous = NORMAL');
+
+db.exec(`
+  CREATE TABLE IF NOT EXISTS documents (
+    key        TEXT PRIMARY KEY,
+    value      TEXT NOT NULL,
+    updated_at TEXT NOT NULL
+  );
+  CREATE TABLE IF NOT EXISTS history (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    key         TEXT NOT NULL,
+    value       TEXT NOT NULL,
+    replaced_at TEXT NOT NULL
+  );
+`);
+
+function getDoc(key) {
+  const row = db.prepare('SELECT value, updated_at FROM documents WHERE key = ?').get(key);
+  if (!row) return null;
+  return { value: JSON.parse(row.value), updatedAt: row.updated_at };
+}
+
+// ponytail: history wächst unbegrenzt. Bei Single-User/kleinen Docs jahrelang egal.
+// Pruning (z.B. > 500 Einträge pro key löschen) nachrüsten, falls es je wächst.
+function putDoc(key, value, now) {
+  const json = JSON.stringify(value);
+  const tx = db.transaction(() => {
+    const existing = db.prepare('SELECT value FROM documents WHERE key = ?').get(key);
+    if (existing) {
+      db.prepare('INSERT INTO history (key, value, replaced_at) VALUES (?, ?, ?)')
+        .run(key, existing.value, now);
+    }
+    db.prepare(`
+      INSERT INTO documents (key, value, updated_at) VALUES (?, ?, ?)
+      ON CONFLICT(key) DO UPDATE SET value = excluded.value, updated_at = excluded.updated_at
+    `).run(key, json, now);
+  });
+  tx();
+}
+
+module.exports = { db, getDoc, putDoc, DB_PATH, DATA_DIR };

server/db.test.js

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+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+
+// DATA_DIR auf Temp umbiegen, BEVOR db.js geladen wird
+process.env.DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), 'dp-db-'));
+const { getDoc, putDoc } = require('./db');
+
+test('putDoc/getDoc Roundtrip', () => {
+  putDoc('employees', ['Max', 'Anna'], '2026-07-07T10:00:00.000Z');
+  const doc = getDoc('employees');
+  assert.deepStrictEqual(doc.value, ['Max', 'Anna']);
+  assert.strictEqual(doc.updatedAt, '2026-07-07T10:00:00.000Z');
+});
+
+test('getDoc für unbekannten Key liefert null', () => {
+  assert.strictEqual(getDoc('gibtsnicht'), null);
+});
+
+test('putDoc snapshottet den alten Wert in history', () => {
+  const { db } = require('./db');
+  putDoc('duties', { v: 1 }, '2026-07-07T10:00:00.000Z');
+  putDoc('duties', { v: 2 }, '2026-07-07T11:00:00.000Z');
+  const rows = db.prepare("SELECT value FROM history WHERE key = 'duties' ORDER BY id").all();
+  assert.strictEqual(rows.length, 1);
+  assert.deepStrictEqual(JSON.parse(rows[0].value), { v: 1 });
+});