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/routes/wohnungen.py

@@ -128,12 +128,20 @@ def _wohnungen_context(user) -> dict:
             "can_apply": allowed and not already_applied,
             "is_running": is_running,
         })
+    # Bezirke the user has narrowed to (CSV). Empty = no district filter, in
+    # which case the map's "excluded districts" overlay stays off entirely.
+    try:
+        selected_districts_csv = (filters_row["districts"] if filters_row else "") or ""
+    except (KeyError, IndexError, TypeError):
+        selected_districts_csv = ""
+
     return {
         "flats": flats_view,
         "rejected_flats": rejected_view,
         "filtered_out_flats": filtered_out_view,
         "partner": partner_info,
         "map_points": map_points,
+        "selected_districts_csv": selected_districts_csv,
         "has_filters": _has_filters(filters_row),
         "alert_label": alert_label,
         "alert_chip": alert_chip,