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/berlin_districts.py

@@ -0,0 +1,120 @@
+"""Berlin-Bezirke taxonomy + PLZ lookup.
+
+The user-visible taxonomy is the 12 Berlin Bezirke. We derive the Bezirk
+from the flat's freeform address by extracting the 5-digit PLZ and
+looking it up in PLZ_TO_DISTRICT.
+
+Some PLZs straddle two Bezirke in reality. Each is mapped to its
+dominant Bezirk — close enough for a list filter. Unmapped PLZs / flats
+without a parseable address resolve to None; the filter treats those as
+"unknown" and excludes them when any Bezirk filter is active.
+"""
+import re
+
+DISTRICTS: list[str] = [
+    "Mitte",
+    "Friedrichshain-Kreuzberg",
+    "Pankow",
+    "Charlottenburg-Wilmersdorf",
+    "Spandau",
+    "Steglitz-Zehlendorf",
+    "Tempelhof-Schöneberg",
+    "Neukölln",
+    "Treptow-Köpenick",
+    "Marzahn-Hellersdorf",
+    "Lichtenberg",
+    "Reinickendorf",
+]
+
+_DISTRICT_PLZ: dict[str, tuple[str, ...]] = {
+    "Mitte": (
+        "10115", "10117", "10119", "10178", "10179",
+        "10551", "10553", "10555", "10557", "10559",
+        "13347", "13349", "13351", "13353", "13355", "13357", "13359",
+    ),
+    "Friedrichshain-Kreuzberg": (
+        "10243", "10245", "10247", "10249",
+        "10961", "10963", "10965", "10967", "10969",
+        "10997", "10999",
+    ),
+    "Pankow": (
+        "10405", "10407", "10409",
+        "10435", "10437", "10439",
+        "13086", "13088", "13089",
+        "13125", "13127", "13129",
+        "13156", "13158", "13159",
+        "13187", "13189",
+    ),
+    "Charlottenburg-Wilmersdorf": (
+        "10585", "10587", "10589",
+        "10623", "10625", "10627", "10629",
+        "10707", "10709", "10711", "10713", "10715", "10717", "10719",
+        "10789",
+        "14050", "14052", "14053", "14055", "14057", "14059",
+        "14193", "14195", "14197", "14199",
+    ),
+    "Spandau": (
+        "13581", "13583", "13585", "13587", "13589",
+        "13591", "13593", "13595", "13597", "13599",
+        "14089",
+    ),
+    "Steglitz-Zehlendorf": (
+        "12157", "12159", "12161", "12163", "12165", "12167", "12169",
+        "12203", "12205", "12207", "12209",
+        "12247", "12249",
+        "14109", "14129",
+        "14163", "14165", "14167", "14169",
+    ),
+    "Tempelhof-Schöneberg": (
+        "10777", "10779", "10781", "10783", "10785", "10787",
+        "10823", "10825", "10827", "10829",
+        "12099", "12101", "12103", "12105", "12107", "12109",
+        "12277", "12279",
+        "12305", "12307", "12309",
+    ),
+    "Neukölln": (
+        "12043", "12045", "12047", "12049",
+        "12051", "12053", "12055", "12057", "12059",
+        "12347", "12349",
+        "12351", "12353", "12355", "12357", "12359",
+    ),
+    "Treptow-Köpenick": (
+        "12435", "12437", "12439",
+        "12459", "12487", "12489",
+        "12524", "12526", "12527",
+        "12555", "12557", "12559",
+        "12587", "12589",
+    ),
+    "Marzahn-Hellersdorf": (
+        "12619", "12621", "12623", "12627", "12629",
+        "12679", "12681", "12683", "12685", "12687", "12689",
+    ),
+    "Lichtenberg": (
+        "10315", "10317", "10318",
+        "10365", "10367", "10369",
+        "13051", "13053", "13055", "13057", "13059",
+    ),
+    "Reinickendorf": (
+        "13403", "13405", "13407", "13409",
+        "13435", "13437", "13439",
+        "13465", "13467", "13469",
+        "13503", "13505", "13507", "13509",
+    ),
+}
+
+PLZ_TO_DISTRICT: dict[str, str] = {
+    plz: district
+    for district, plzs in _DISTRICT_PLZ.items()
+    for plz in plzs
+}
+
+_PLZ_RE = re.compile(r"\b(\d{5})\b")
+
+
+def district_for_address(address: str | None) -> str | None:
+    if not address:
+        return None
+    m = _PLZ_RE.search(address)
+    if not m:
+        return None
+    return PLZ_TO_DISTRICT.get(m.group(1))