feat(filter): Berlin-Bezirk filter in Einstellungen

Adds a collapsible 12-Bezirk checkbox list to the filter tab. The UI
speaks Bezirke; internally the match runs on the PLZ extracted from
flat.address and resolved to a dominant Bezirk via a curated 187-PLZ
map (berlin_districts.py).

- Migration 0011 adds user_filters.districts (CSV of selected names)
- Empty stored value = no filter = all Bezirke ticked in the UI.
  Submitting "all ticked" or "none ticked" both normalise to empty
  so the defaults and the nuclear state mean the same thing.
- When a Bezirk filter is active, flats with an unknown/unmapped PLZ
  are excluded — if the user bothered to narrow by district, sneaking
  in unplaceable flats would be the wrong default.
- Filter summary on the Wohnungen page shows "N Bezirke" so it's
  visible the filter is active.

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

web/templates/_settings_filter.html

@@ -43,6 +43,30 @@
             {% endfor %}
         </select>
     </div>
+    <div class="col-span-2 md:col-span-3">
+        <details class="border border-soft rounded p-3"
+                 {% if selected_districts|length != all_districts|length %}open{% endif %}>
+            <summary class="cursor-pointer font-medium text-sm select-none">
+                Stadtteile
+                <span class="text-slate-500 text-xs ml-1">
+                    {% if selected_districts|length == all_districts|length %}alle{% else %}{{ selected_districts|length }} / {{ all_districts|length }}{% endif %}
+                </span>
+            </summary>
+            <div class="grid grid-cols-2 md:grid-cols-3 gap-2 mt-3">
+                {% for d in all_districts %}
+                <label class="flex items-center gap-2 text-sm">
+                    <input type="checkbox" name="districts" value="{{ d }}"
+                           {% if d in selected_districts %}checked{% endif %}>
+                    <span>{{ d }}</span>
+                </label>
+                {% endfor %}
+            </div>
+            <p class="text-xs text-slate-500 mt-2">
+                Standard: alle angekreuzt. Wohnungen ohne erkennbare Berliner PLZ werden
+                ausgeblendet, sobald du mindestens einen Haken entfernst.
+            </p>
+        </details>
+    </div>
     <div class="col-span-2 md:col-span-3">
         <button class="btn btn-primary" type="submit">Filter speichern</button>
     </div>