feat: add preistraeger placeholder image and related database migrations

- Introduced a new SVG placeholder image for preistraeger without an image.
- Created migration scripts to add download zip and gallery features to the preistraeger table.
- Added site settings for preistraeger placeholder image in the database.
- Implemented a utility function to retrieve the preistraeger placeholder image URL in the SPA.

src/spa/preistraegerPlaceholder.ts

@@ -0,0 +1,11 @@
+import { mediaUrl } from '@/spa/cmsMediaField'
+import { useCmsRoute } from '@/spa/cmsRoute'
+import { defaultSiteSettings } from '@/spa/siteSettings'
+
+export const PREISTRAEGER_PLACEHOLDER_IMAGE = defaultSiteSettings.preistraegerPlaceholderImageUrl
+
+export function usePreistraegerPlaceholderImage() {
+  const siteSettings = useCmsRoute()?.siteSettings || defaultSiteSettings
+
+  return mediaUrl(siteSettings.preistraegerPlaceholderImage, siteSettings.preistraegerPlaceholderImageUrl)
+}