fix: round €/m² in Telegram, drop "Bilder nachladen" admin button, fix lightbox visibility

- notifications: round sqm_price to whole € in Telegram match messages
  (was emitting raw float like "12.345614 €/m²").

- wohnungen: remove the admin-only "Bilder nachladen (N)" button. It
  flickered into view whenever a freshly-scraped flat was still in
  pending state, which was effectively random from the user's point of
  view, and the manual backfill it triggered isn't needed anymore — new
  flats are auto-enriched at scrape time. Also drops the dead helpers
  it was the sole caller of: enrichment.kick_backfill,
  enrichment._backfill_runner, db.flats_needing_enrichment,
  db.enrichment_counts.

- lightbox: the modal didn't appear because Tailwind's Play CDN injects
  its own .hidden { display: none } rule at runtime, which kept fighting
  our class toggle. Switch the show/hide to inline style.display so no
  external stylesheet can mask it. Single-class .lightbox now only owns
  the layout — the initial-hidden state is on the element via
  style="display:none".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/base.html

@@ -20,7 +20,7 @@
 {% block body %}{% endblock %}
 
 {# Image lightbox — global so any flat-gallery on any page reuses the same modal. #}
-<div id="lazyflat-lightbox" class="lightbox hidden" aria-hidden="true" role="dialog" aria-label="Bildansicht">
+<div id="lazyflat-lightbox" class="lightbox" style="display:none" aria-hidden="true" role="dialog" aria-label="Bildansicht">
     <button class="lightbox-close" type="button" aria-label="Schließen" data-lightbox-close>
         <svg viewBox="0 0 24 24" width="22" height="22" fill="none" stroke="currentColor" stroke-width="2.4" stroke-linecap="round" stroke-linejoin="round"><line x1="6" y1="6" x2="18" y2="18"/><line x1="18" y1="6" x2="6" y2="18"/></svg>
     </button>