feat(calendar): expand recurring events in GET /calendar and /upcoming

expandRecurringEvents() iterates from the event's original start date,
generating all occurrences within the requested window using the existing
nextOccurrence() service (max 1000 iterations). The SQL query is extended
to also fetch recurring events that started before the window. Event
duration is preserved across instances. Virtual instances carry
is_recurring_instance=1 and are shown with a repeat icon in the agenda
view. /upcoming expands across a 90-day forward window.

Closes BL-01.

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

server/routes/calendar.js

@@ -15,8 +15,71 @@ const appleCalendar  = require('../services/apple-calendar');
 const { requireAdmin } = require('../auth');
 const { str, color, datetime, rrule, collectErrors, MAX_TITLE, MAX_TEXT, DATE_RE, DATETIME_RE } = require('../middleware/validate');
 
+const { nextOccurrence } = require('../services/recurrence');
+
 const VALID_SOURCES = ['local', 'google', 'apple'];
 
+// --------------------------------------------------------
+// RRULE-Expansion: alle Vorkommen eines wiederkehrenden Events
+// innerhalb [from, to] generieren (inklusive beider Grenzen).
+// --------------------------------------------------------
+
+/**
+ * @param {object[]} events  Rohe DB-Events (können recurrence_rule haben)
+ * @param {string}   from    YYYY-MM-DD
+ * @param {string}   to      YYYY-MM-DD
+ * @returns {object[]}  Expandiertes, sortiertes Array
+ */
+function expandRecurringEvents(events, from, to) {
+  const result = [];
+
+  for (const event of events) {
+    if (!event.recurrence_rule) {
+      result.push(event);
+      continue;
+    }
+
+    // Dauer des Events in ms (für End-Zeit-Berechnung der Instanzen)
+    const startMs    = new Date(event.start_datetime).getTime();
+    const endMs      = event.end_datetime ? new Date(event.end_datetime).getTime() : null;
+    const durationMs = endMs !== null ? endMs - startMs : null;
+
+    // Original-Zeit-Teil erhalten (z.B. 'T14:30:00' oder '' bei All-Day)
+    const timeSuffix = event.start_datetime.slice(10);
+
+    let currentDate = event.start_datetime.slice(0, 10); // YYYY-MM-DD
+    let iterations  = 0;
+    const MAX_ITER  = 1000; // Sicherheitsgrenze
+
+    while (currentDate <= to && iterations < MAX_ITER) {
+      iterations++;
+
+      if (currentDate >= from) {
+        const newStart = currentDate + timeSuffix;
+        let newEnd = event.end_datetime;
+        if (durationMs !== null) {
+          newEnd = new Date(new Date(newStart).getTime() + durationMs)
+            .toISOString()
+            .replace('.000Z', 'Z');
+        }
+
+        result.push({
+          ...event,
+          start_datetime:       newStart,
+          end_datetime:         newEnd,
+          is_recurring_instance: currentDate !== event.start_datetime.slice(0, 10) ? 1 : 0,
+        });
+      }
+
+      const next = nextOccurrence(currentDate, event.recurrence_rule);
+      if (!next || next <= currentDate) break;
+      currentDate = next;
+    }
+  }
+
+  return result.sort((a, b) => a.start_datetime.localeCompare(b.start_datetime));
+}
+
 // --------------------------------------------------------
 // GET /api/v1/calendar
 // Termine in einem Datumsbereich abrufen.
@@ -46,11 +109,14 @@ router.get('/', (req, res) => {
       LEFT JOIN users u_assigned ON u_assigned.id = e.assigned_to
       LEFT JOIN users u_created  ON u_created.id  = e.created_by
       WHERE (
-        DATE(e.start_datetime) <= ? AND
-        (e.end_datetime IS NULL OR DATE(e.end_datetime) >= ?)
+        (e.recurrence_rule IS NULL AND
+          DATE(e.start_datetime) <= ? AND
+          (e.end_datetime IS NULL OR DATE(e.end_datetime) >= ?))
+        OR
+        (e.recurrence_rule IS NOT NULL AND DATE(e.start_datetime) <= ?)
       )
     `;
-    const params = [to, from];
+    const params = [to, from, to];
 
     if (req.query.assigned_to) {
       sql += ' AND e.assigned_to = ?';
@@ -64,7 +130,8 @@ router.get('/', (req, res) => {
 
     sql += ' ORDER BY e.start_datetime ASC, e.all_day DESC';
 
-    const events = db.get().prepare(sql).all(...params);
+    const rawEvents = db.get().prepare(sql).all(...params);
+    const events    = expandRecurringEvents(rawEvents, from, to);
     res.json({ data: events, from, to });
   } catch (err) {
     console.error('[calendar/GET /]', err);
@@ -80,21 +147,30 @@ router.get('/', (req, res) => {
 // --------------------------------------------------------
 router.get('/upcoming', (req, res) => {
   try {
-    const limit = Math.min(parseInt(req.query.limit, 10) || 5, 20);
-    const now   = new Date().toISOString();
+    const limit   = Math.min(parseInt(req.query.limit, 10) || 5, 20);
+    const nowDate = new Date().toISOString().slice(0, 10);
+    // Fenster: heute bis 90 Tage voraus (für Wiederholungs-Expansion)
+    const future  = new Date(Date.now() + 90 * 24 * 60 * 60 * 1000).toISOString().slice(0, 10);
 
-    const events = db.get().prepare(`
+    const rawEvents = db.get().prepare(`
       SELECT e.*,
              u_assigned.display_name AS assigned_name,
              u_assigned.avatar_color AS assigned_color
       FROM calendar_events e
       LEFT JOIN users u_assigned ON u_assigned.id = e.assigned_to
-      WHERE e.start_datetime >= ?
+      WHERE (
+        (e.recurrence_rule IS NULL AND DATE(e.start_datetime) BETWEEN ? AND ?)
+        OR
+        (e.recurrence_rule IS NOT NULL AND DATE(e.start_datetime) <= ?)
+      )
       ORDER BY e.start_datetime ASC
-      LIMIT ?
-    `).all(now, limit);
+    `).all(nowDate, future, future);
 
-    res.json({ data: events });
+    const expanded = expandRecurringEvents(rawEvents, nowDate, future)
+      .filter((e) => e.start_datetime >= new Date().toISOString())
+      .slice(0, limit);
+
+    res.json({ data: expanded });
   } catch (err) {
     console.error('[calendar/GET /upcoming]', err);
     res.status(500).json({ error: 'Interner Fehler', code: 500 });