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/db.py

@@ -270,6 +270,11 @@ MIGRATIONS: list[str] = [
     ALTER TABLE flats ADD COLUMN offline_at TEXT;
     CREATE INDEX IF NOT EXISTS idx_flats_offline ON flats(offline_at);
     """,
+    # 0011: per-user Berlin-Bezirk filter. CSV of selected Bezirk names;
+    # empty = no filter (= all Bezirke match).
+    """
+    ALTER TABLE user_filters ADD COLUMN districts TEXT NOT NULL DEFAULT '';
+    """,
 ]
 
 
@@ -463,7 +468,7 @@ def get_filters(user_id: int) -> sqlite3.Row:
 def update_filters(user_id: int, data: dict) -> None:
     _ensure_user_rows(user_id)
     allowed = {"rooms_min", "rooms_max", "max_rent", "min_size",
-               "wbs_required", "max_age_hours"}
+               "wbs_required", "max_age_hours", "districts"}
     clean = {k: data.get(k) for k in allowed if k in data}
     if not clean:
         return