feat: make /imkerei and /herkunft hero images admin-editable

Both pages are the ones most directly meant to give customers "ein
gutes Bild von der Imkerei" (per the active /goal), but their hero
images were still hardcoded PlaceholderImage tiles with no way to
ever swap in a real photo — unlike every homepage section, which
already went through this exact upload flow.

Added imkerei.image / herkunft.image to the content registry (new
"pages" group, since neither belongs under "Homepage") and switched
both pages from PlaceholderImage to ContentImage + getContentValue,
the same pattern already proven on the homepage sections. Falls back
to the identical placeholder tile until a photo is uploaded, so this
is a pure addition with no visual change until the owner uploads one.

Verified: tsc clean, 151/151 tests pass, both pages still render
correctly (curl 200 + screenshot) with the placeholder fallback.
Could not verify the admin upload UI end-to-end live — the local
admin account has 2FA enabled and I don't have the second factor;
confidence instead comes from reusing the identical, already-verified
AdminImageField/ContentImage wiring used for the four home.*.image
fields, plus generic (group-agnostic) admin field loading confirmed
by reading admin/inhalte/page.tsx.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/app/admin/inhalte/page.tsx

@@ -33,6 +33,7 @@ export default async function AdminInhaltePage(props: PageProps<"/admin/inhalte"
   const groups: { key: ContentFieldDef["group"]; heading: string }[] = [
     { key: "company", heading: t.adminContentPage.groupCompany },
     { key: "home", heading: t.adminContentPage.groupHome },
+    { key: "pages", heading: t.adminContentPage.groupPages },
     { key: "social", heading: t.adminContentPage.groupSocial },
     { key: "legal", heading: t.adminContentPage.groupLegal },
   ];