feat: Serientermin-Bearbeitung: Nur diesen Termin oder Serie

Klick auf einen wiederkehrenden Termin oeffnet zuerst einen Dialog:
"Nur diesen Termin" oder "Ganze Serie".

* Serie: bearbeitet den Master wie bisher
* Nur dieser: fuegt EXDATE fuer das geklickte Datum zum Master
  hinzu und legt einen eigenstaendigen Ersatz-Termin mit den
  bearbeiteten Daten an

Backend:
* CalendarEvent.exdates speichert Ausnahmedaten kommasepariert
* POST /events/<id>/exception fuegt EXDATE hinzu, erstellt
  optional das Replacement-Event mit frischer UID
* _build_vevent schreibt jetzt EXDATE-Zeilen in die ical_data,
  sodass CalDAV-Clients die Ausnahmen auch sehen werden

Frontend:
* FullCalendar rrule-Plugin bekommt die exdate-Liste und blendet
  die uebersprungenen Tage aus
* Drag & Drop verschiebt weiterhin die ganze Serie (Shortcut -
  fuer Einzelverschiebung Termin anklicken und bearbeiten)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/CalendarView.vue

@@ -186,6 +186,16 @@
       </template>
     </Dialog>
 
+    <!-- Scope picker for recurring events -->
+    <Dialog v-model:visible="showScopeDialog" header="Serientermin bearbeiten" modal :style="{ width: '440px' }">
+      <p>Dieser Termin gehoert zu einer Serie.<br>Was moechtest du bearbeiten?</p>
+      <template #footer>
+        <Button label="Abbrechen" text @click="showScopeDialog = false" />
+        <Button label="Nur diesen Termin" outlined @click="openEditScope('occurrence')" />
+        <Button label="Ganze Serie" @click="openEditScope('series')" />
+      </template>
+    </Dialog>
+
     <Dialog v-model:visible="confirmDeleteCal" header="Kalender loeschen" modal :style="{ width: '400px' }">
       <p>Moechtest du den Kalender <strong>{{ selectedCal?.name }}</strong> mit allen Terminen loeschen?</p>
       <template #footer>
@@ -271,6 +281,11 @@ const sharePermission = ref('read')
 const icalPassword = ref('')
 const confirmDeleteEvent = ref(false)
 const confirmDeleteCal = ref(false)
+const showScopeDialog = ref(false)
+const pendingScopeEvent = ref(null)
+const pendingScopeOccurrence = ref(null)
+// 'series' (edit master), 'occurrence' (split off single date), or null (non-recurring)
+const currentEditScope = ref(null)
 const permOptions = [{ label: 'Lesen', value: 'read' }, { label: 'Lesen+Schreiben', value: 'readwrite' }]
 
 const ownCalendars = computed(() => calendars.value.filter(c => c.permission === 'owner'))
@@ -354,12 +369,14 @@ function toFcEvent(e, cal) {
     extendedProps: { ...e, _cal: cal },
   }
   if (e.recurrence_rule) {
-    // FullCalendar rrule plugin consumes the RRULE together with dtstart
     fc.rrule = {
       freq: extractFreq(e.recurrence_rule),
       dtstart: e.dtstart,
       ...parseRRule(e.recurrence_rule),
     }
+    if (e.exdates && e.exdates.length) {
+      fc.exdate = e.exdates
+    }
     if (e.dtend && e.dtstart) {
       const ms = new Date(e.dtend).getTime() - new Date(e.dtstart).getTime()
       if (ms > 0) fc.duration = msToDuration(ms)
@@ -443,9 +460,47 @@ function loadRRuleIntoForm(rrule) {
 
 function onEventClick(info) {
   const e = info.event.extendedProps
+  if (e.recurrence_rule) {
+    pendingScopeEvent.value = e
+    pendingScopeOccurrence.value = info.event.start
+    showScopeDialog.value = true
+    return
+  }
+  currentEditScope.value = null
   openEditEvent(e)
 }
 
+function openEditScope(scope) {
+  showScopeDialog.value = false
+  const e = pendingScopeEvent.value
+  const occ = pendingScopeOccurrence.value
+  if (scope === 'series') {
+    currentEditScope.value = 'series'
+    openEditEvent(e)
+  } else {
+    // "Nur diesen Termin" - form starts from the clicked occurrence's
+    // start/end, non-recurring. Save will add an EXDATE on the master
+    // plus create a standalone event.
+    currentEditScope.value = 'occurrence'
+    editingEvent.value = e
+    const durationMs = e.dtend ? new Date(e.dtend) - new Date(e.dtstart) : 3600000
+    const occEnd = new Date(occ.getTime() + durationMs)
+    eventForm.value = {
+      summary: e.summary,
+      description: e.description || '',
+      location: e.location || '',
+      calendar_id: e.calendar_id,
+      dtstart: toLocalISO(occ, e.all_day),
+      dtend: toLocalISO(occEnd, e.all_day),
+      all_day: e.all_day,
+    }
+    loadRRuleIntoForm('')
+    // Freeze recurrence editor - this is a single instance
+    recurFreq.value = 'none'
+    showEventDialog.value = true
+  }
+}
+
 function onDateSelect(info) {
   openNewEvent(info.start, info.end, info.allDay)
 }
@@ -534,12 +589,28 @@ async function saveEvent() {
     calendar_id: eventForm.value.calendar_id,
   }
   try {
-    if (editingEvent.value) {
+    if (currentEditScope.value === 'occurrence' && editingEvent.value) {
+      // Split single occurrence off the series: add EXDATE + create replacement
+      await apiClient.post(`/events/${editingEvent.value.id}/exception`, {
+        occurrence_date: toLocalISO(pendingScopeOccurrence.value, editingEvent.value.all_day),
+        replacement: {
+          summary: payload.summary,
+          description: payload.description,
+          location: payload.location,
+          dtstart: payload.dtstart,
+          dtend: payload.dtend,
+          all_day: payload.all_day,
+        },
+      })
+    } else if (editingEvent.value) {
       await apiClient.put(`/events/${editingEvent.value.id}`, payload)
     } else {
       await apiClient.post(`/calendars/${payload.calendar_id}/events`, payload)
     }
     showEventDialog.value = false
+    currentEditScope.value = null
+    pendingScopeEvent.value = null
+    pendingScopeOccurrence.value = null
     refreshEvents()
   } catch (err) {
     toast.add({ severity: 'error', summary: 'Fehler', detail: err.response?.data?.error, life: 5000 })