feat: Social-Media-Bild-Generator für Kalender-Termine

Neuer Endpoint /api/social/calendar/[slug] rendert Termin via satori +
resvg zu PNG. Formate: og (1200x630) und square (1080x1080).
Gebrandetes Layout mit Datum, Titel, Ort. Inter-Font lazy von gstatic,
10min In-Memory-Cache pro Slug+Format.

CalendarBlock: zwei Download-Buttons pro Termin (Bild quer / quadr.).

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

scripts/generate-icons.mjs

@@ -67,6 +67,9 @@ const EXTRA_ICONS = [
   'information-outline',
   // OrganisationsMapBlock: activate-overlay
   'cursor-default-click-outline',
+  // CalendarBlock: social-image buttons
+  'image-outline',
+  'crop-square',
 ];
 
 const STATIC_PATTERN = /icon="mdi:([a-z0-9-]+)"/g;