experiment: shade excluded Bezirke on map (faint yellow overlay)

Tints districts the user's Bezirk filter has EXCLUDED in light yellow
(#fde68a, fillOpacity 0.35) so the active selection is obvious from
the map alone — and you can see at a glance whether a "rausgefiltert"
flat fell in a no-go district. When no district filter is set the
overlay stays off entirely (nothing is excluded).

Wiring:
- Berlin Bezirke GeoJSON checked in at web/static/berlin-districts.geojson
  (12 features, name property matches our DISTRICTS list 1:1, props
  stripped down to {name} and minified — 312KB raw, ~80KB gzipped).
- Route exposes the user's selected_districts_csv to the template.
- The flats-map-data <script> carries it on data-selected-districts so
  it flows in alongside csrf and the marker payload.
- map.js fetches the GeoJSON once (cache normally), keeps the layer in
  a module-level reference, and re-styles it via setStyle() on every
  swap (cheap). Marker layer is kicked to the front so pins always
  paint above the shaded polygons. fingerprintOf now also folds in
  the selected-districts CSV so a Bezirk-only filter change still
  triggers a re-render.

Branch only — kept off main while we see if this reads well.

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

web/templates/_wohnungen_body.html

@@ -103,7 +103,9 @@
         <div id="flats-map" hx-preserve="true"></div>
     </div>
 </section>
-<script id="flats-map-data" type="application/json" data-csrf="{{ csrf }}">{{ map_points | tojson }}</script>
+<script id="flats-map-data" type="application/json"
+        data-csrf="{{ csrf }}"
+        data-selected-districts="{{ selected_districts_csv }}">{{ map_points | tojson }}</script>
 
 <!-- Liste -->
 <section class="view-list card">