feat(budget): auto-generate recurring entry instances per month

Adds schema migration v3 (recurrence_parent_id column + budget_recurrence_skipped
table). On every GET /api/v1/budget, the server checks all recurring originals
(is_recurring=1, no parent) and creates missing instances for the requested month
using the same day-of-month (clamped to the last day). Deleted instances are
recorded in budget_recurrence_skipped so they are not recreated on the next visit.
Generated instances are shown with a ↩ indicator in the transaction list.

Closes BL-05.

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

BACKLOG.md

@@ -70,7 +70,7 @@ Die Sync-Services `server/services/google-calendar.js` und `server/services/appl
 
 ### BL-05 — Budget: Wiederkehrende Buchungen automatisch generieren
 
-**Status:** Offen
+**Status:** Erledigt (v0.3.0)
 **Aufwand:** S (1–2 Tage)
 
 Das Budget-Formular hat eine „Wiederkehrend"-Checkbox und speichert `is_recurring = 1`. Es fehlt jedoch die automatische Generierung der Folgebuchungen. Derzeit muss der Nutzer jede Buchung manuell eintragen.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Budget: recurring entries auto-generate instances for each viewed month; instances deleted by the user are skipped permanently via `budget_recurrence_skipped` table; generated instances are marked with ↩ in the transaction list
 - Budget: month-over-month comparison in summary cards — each card (Einnahmen, Ausgaben, Saldo) shows a trend line (▲/▼ + delta amount vs. previous month); previous month summary is fetched in parallel with current month
 
 ## [0.2.1] - 2026-03-30

public/pages/budget.js

@@ -267,13 +267,14 @@ function renderEntries() {
     const indClass  = isIncome ? 'budget-entry__indicator--income' : 'budget-entry__indicator--expenses';
     const sign      = isIncome ? '+' : '';
     const date      = formatEntryDate(e.date);
+    const recurTag  = e.is_recurring ? ' 🔁' : (e.recurrence_parent_id ? ' ↩' : '');
 
     return `
       <div class="budget-entry" data-id="${e.id}">
         <div class="budget-entry__indicator ${indClass}"></div>
         <div class="budget-entry__body">
           <div class="budget-entry__title">${escHtml(e.title)}</div>
-          <div class="budget-entry__meta">${date} · ${escHtml(e.category)}${e.is_recurring ? ' 🔁' : ''}</div>
+          <div class="budget-entry__meta">${date} · ${escHtml(e.category)}${recurTag}</div>
         </div>
         <div class="budget-entry__amount ${amtClass}">${sign}${formatAmount(e.amount)}</div>
         <button class="budget-entry__delete" data-action="delete" data-id="${e.id}" aria-label="Eintrag löschen">

server/db.js

@@ -278,6 +278,22 @@ const MIGRATIONS = [
       CREATE INDEX IF NOT EXISTS idx_calendar_external_id ON calendar_events(external_calendar_id);
     `,
   },
+  {
+    version: 3,
+    description: 'Wiederkehrende Budget-Einträge: parent-Referenz und Skip-Tabelle',
+    up: `
+      ALTER TABLE budget_entries ADD COLUMN recurrence_parent_id INTEGER
+        REFERENCES budget_entries(id) ON DELETE SET NULL;
+
+      CREATE TABLE IF NOT EXISTS budget_recurrence_skipped (
+        parent_id INTEGER NOT NULL REFERENCES budget_entries(id) ON DELETE CASCADE,
+        month     TEXT    NOT NULL,
+        PRIMARY KEY (parent_id, month)
+      );
+
+      CREATE INDEX IF NOT EXISTS idx_budget_parent ON budget_entries(recurrence_parent_id);
+    `,
+  },
 ];
 
 /**

server/routes/budget.js

@@ -11,6 +11,56 @@ const router  = express.Router();
 const db      = require('../db');
 const { str, oneOf, date, num, rrule, collectErrors, MAX_TITLE, MONTH_RE } = require('../middleware/validate');
 
+// --------------------------------------------------------
+// Wiederkehrende Einträge: fehlende Instanzen für einen Monat erzeugen
+// --------------------------------------------------------
+
+/**
+ * Erstellt fehlende Instanzen wiederkehrender Budget-Einträge für den angefragten Monat.
+ * Läuft idempotent — bereits vorhandene oder explizit übersprungene Instanzen werden ignoriert.
+ * @param {import('better-sqlite3').Database} database
+ * @param {string} month  YYYY-MM
+ */
+function generateRecurringInstances(database, month) {
+  const [y, m] = month.split('-').map(Number);
+  const monthStart = `${month}-01`;
+  const monthEnd   = `${month}-31`;
+
+  // Alle Serien-Originale, die vor diesem Monat begonnen haben
+  const originals = database.prepare(`
+    SELECT * FROM budget_entries
+    WHERE is_recurring = 1 AND recurrence_parent_id IS NULL
+      AND strftime('%Y-%m', date) < ?
+  `).all(month);
+
+  for (const orig of originals) {
+    // Übersprungener Monat?
+    const skipped = database.prepare(
+      'SELECT 1 FROM budget_recurrence_skipped WHERE parent_id = ? AND month = ?'
+    ).get(orig.id, month);
+    if (skipped) continue;
+
+    // Instanz schon vorhanden?
+    const existing = database.prepare(`
+      SELECT id FROM budget_entries
+      WHERE recurrence_parent_id = ? AND date BETWEEN ? AND ?
+    `).get(orig.id, monthStart, monthEnd);
+    if (existing) continue;
+
+    // Datum berechnen: gleicher Tag, am letzten Tag des Monats gekappt
+    const origDay    = parseInt(orig.date.split('-')[2], 10);
+    const lastDay    = new Date(y, m, 0).getDate();
+    const instanceDay = Math.min(origDay, lastDay);
+    const instanceDate = `${month}-${String(instanceDay).padStart(2, '0')}`;
+
+    database.prepare(`
+      INSERT INTO budget_entries
+        (title, amount, category, date, is_recurring, recurrence_parent_id, created_by)
+      VALUES (?, ?, ?, ?, 0, ?, ?)
+    `).run(orig.title, orig.amount, orig.category, instanceDate, orig.id, orig.created_by);
+  }
+}
+
 const VALID_CATEGORIES = [
   'Lebensmittel', 'Miete', 'Versicherung', 'Mobilität',
   'Freizeit', 'Kleidung', 'Gesundheit', 'Bildung', 'Sonstiges',
@@ -144,6 +194,8 @@ router.get('/', (req, res) => {
     if (!MONTH_RE.test(month))
       return res.status(400).json({ error: 'month muss YYYY-MM sein', code: 400 });
 
+    generateRecurringInstances(db.get(), month);
+
     const from   = `${month}-01`;
     const to     = `${month}-31`;
     let sql      = `
@@ -268,9 +320,19 @@ router.put('/:id', (req, res) => {
 router.delete('/:id', (req, res) => {
   try {
     const id    = parseInt(req.params.id, 10);
-    const result = db.get().prepare('DELETE FROM budget_entries WHERE id = ?').run(id);
-    if (result.changes === 0)
-      return res.status(404).json({ error: 'Eintrag nicht gefunden', code: 404 });
+    const entry = db.get().prepare('SELECT * FROM budget_entries WHERE id = ?').get(id);
+    if (!entry) return res.status(404).json({ error: 'Eintrag nicht gefunden', code: 404 });
+
+    db.get().prepare('DELETE FROM budget_entries WHERE id = ?').run(id);
+
+    // Wenn eine Instanz gelöscht wird: Monat als übersprungen markieren
+    if (entry.recurrence_parent_id) {
+      const month = entry.date.slice(0, 7);
+      db.get().prepare(
+        'INSERT OR IGNORE INTO budget_recurrence_skipped (parent_id, month) VALUES (?, ?)'
+      ).run(entry.recurrence_parent_id, month);
+    }
+
     res.status(204).end();
   } catch (err) {
     console.error('[budget/DELETE /:id]', err);