Birthday sync-device list + guard group-visible against birthday calendar

- new BirthdaySyncDevice table + /api/birthdays/sync-report & /devices so the
  web can show "birthdays come from these devices" (iOS reports its device on
  each Contacts sync)
- settings: reject setting a birthday calendar as the group-visible calendar
  (it can still be shared directly)
- web: Settings > Calendars > Birthdays shows the device list; exclude birthday
  calendars from the group-visible picker

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -297,6 +297,8 @@ const translations = {
     birthday_year_ph: 'Jahr',
     birthday_settings_title: 'Geburtstage',
     birthday_color_hint: 'Farbe und Sichtbarkeit änderst du in der Seitenleiste.',
+    birthday_devices_title: 'Geburtstage kommen von diesen Geräten:',
+    birthday_devices_count: '{n} Geburtstage',
     error_name_url: 'Bitte Name und URL eingeben',
     ical_subscribed: '"{name}" abonniert',
     google_synced: 'Kalender synchronisiert',
@@ -639,6 +641,8 @@ const translations = {
     birthday_year_ph: 'Year',
     birthday_settings_title: 'Birthdays',
     birthday_color_hint: 'Change colour and visibility in the sidebar.',
+    birthday_devices_title: 'Birthdays come from these devices:',
+    birthday_devices_count: '{n} birthdays',
     error_name_url: 'Please enter a name and URL',
     ical_subscribed: '"{name}" subscribed',
     google_synced: 'Calendar synced',