Make omsorgapp's backend URL runtime-configurable, use dedicated API subdomain

- omsorgapp now reads the omsorgCore URL from a runtime env-config.js
  generated by the container entrypoint from OMSORG_CORE_URL, instead of
  only baking it in at image build time - docker-compose.yml pulls
  pre-built images from the registry, so a build-time-only value couldn't
  be changed without a rebuild.
- docker-compose.yml: omsorgCore gets its own subdomain
  (core.omsorg-pflegedienste.de) rather than being proxied under the
  frontend's domain - omsorgWeb never needs browser-side access to it
  anyway (server-side cURL only), and a dedicated API host is more
  future-proof without being any less secure (backend still only bound to
  127.0.0.1). Includes the nginx server-block snippet needed for the new
  subdomain.
- Drop the now-unused OMSORG_CORE_PUBLIC_URL build-arg wiring from the
  Gitea Actions workflow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

omsorgapp/public/env-config.js

@@ -0,0 +1,4 @@
+// Dev-Default (Vite serviert alles unter public/ unverändert unter /) - im Docker-Image wird
+// diese Datei vom Entrypoint (docker-entrypoint.sh) aus der OMSORG_CORE_URL-Env-Var überschrieben,
+// siehe src/api/config.js.
+window.__OMSORG_ENV__ = {};