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.

public/images/preistraeger-placeholder.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1200 900" role="img" aria-labelledby="title desc">
+  <title id="title">Neutraler Preistraeger Platzhalter</title>
+  <desc id="desc">Neutraler Platzhalter fuer Preistraeger ohne eigenes Bild.</desc>
+  <rect width="1200" height="900" fill="#E4E2E3"/>
+  <rect x="84" y="84" width="1032" height="732" fill="#F7F7F8"/>
+  <path d="M84 816 1116 84" stroke="#D0D5DD" stroke-width="2" opacity=".7"/>
+  <path d="M84 84 1116 816" stroke="#D0D5DD" stroke-width="2" opacity=".45"/>
+  <circle cx="600" cy="384" r="112" fill="#C6CBD3"/>
+  <path d="M360 720c22-146 124-224 240-224s218 78 240 224H360z" fill="#B8BEC8"/>
+  <rect x="84" y="84" width="1032" height="732" fill="none" stroke="#D0D5DD" stroke-width="3"/>
+  <rect x="84" y="812" width="1032" height="4" fill="#EFBF04"/>
+  <text x="600" y="782" text-anchor="middle" font-family="IBM Plex Sans, Arial, sans-serif" font-size="36" font-weight="700" letter-spacing="5" fill="#111D55">BMP</text>
+</svg>