fix(sf-08): add euro-1/euro-2 room_bucket for small-format rooms=2 flats

DOM.РФ API classifies euro-format flats (26-50м²) as rooms=2.
_SUPPLY_BATCH_SQL now maps:
  rooms=2 + area<35 → 'euro-1'
  rooms=2 + area<50 → 'euro-2'
before the generic rooms IN (1,2,3) branch.

Audit: 4 140 units rooms=2 + area<35 in domrf_kn_flats were inflating
the supply count for genuine 2-room apartments (median 57м²).
euro buckets have no velocity counterpart in objective_corpus_room_month
so they are filtered out of top_layouts by min_velocity.

Also extends RoomBucket Literal and room_bucket_from_flat() in
layout_signature.py with the new euro-1/euro-2 values.

Closes #271 item 8

backend/app/services/site_finder/best_layouts.py

@@ -143,6 +143,11 @@ _SUPPLY_BATCH_SQL = text("""
         CASE
             WHEN f.is_studio = TRUE OR f.flat_type = 'Квартира-студия' THEN 'studio'
             WHEN f.rooms = 0 THEN 'studio'
+            -- Fix SF-08: euro-форматы — DOM.РФ маркирует малогабаритные квартиры как 2-комн.
+            -- rooms=2 + area<35 → euro-1 (студия с отдельной кухней ~26м²)
+            -- rooms=2 + area<50 → euro-2 (~35-50м², евро-двушка)
+            WHEN f.rooms = 2 AND f.total_area < 35 THEN 'euro-1'
+            WHEN f.rooms = 2 AND f.total_area < 50 THEN 'euro-2'
             WHEN f.rooms IN (1, 2, 3) THEN f.rooms::text
             WHEN f.rooms >= 4 THEN '4+'
             ELSE '1'

backend/app/services/site_finder/layout_signature.py

@@ -7,7 +7,7 @@ from __future__ import annotations
 
 from typing import Literal
 
-RoomBucket = Literal["studio", "1", "2", "3", "4+"]
+RoomBucket = Literal["studio", "euro-1", "euro-2", "1", "2", "3", "4+"]
 AreaBin = Literal["<25", "25-40", "40-60", "60-80", "80-100", "100+"]
 
 
@@ -15,15 +15,18 @@ def room_bucket_from_flat(
     rooms: int | None,
     flat_type: str | None,
     is_studio: bool | None,
+    total_area: float | None = None,
 ) -> RoomBucket:
     """Determine room_bucket из kn_flats полей.
 
     Priority:
     1. is_studio=True OR flat_type='Квартира-студия' → "studio"
-    2. rooms IN (1, 2, 3) → str(rooms)
-    3. rooms >= 4 → "4+"
-    4. rooms = 0 (без is_studio) → "studio"
-    5. fallback на "1" (rooms is None и не studio)
+    2. rooms=0 (без is_studio) → "studio"
+    3. Fix SF-08: rooms=2 + area<35 → "euro-1" (DOM.РФ маркирует малогабаритные как 2-комн)
+    4. Fix SF-08: rooms=2 + area<50 → "euro-2" (евро-двушки 35-50м²)
+    5. rooms IN (1, 2, 3) → str(rooms)
+    6. rooms >= 4 → "4+"
+    7. fallback на "1" (rooms is None и не studio)
     """
     if is_studio is True or flat_type == "Квартира-студия":
         return "studio"
@@ -31,6 +34,11 @@ def room_bucket_from_flat(
         return "1"
     if rooms == 0:
         return "studio"
+    if rooms == 2 and total_area is not None:
+        if total_area < 35.0:
+            return "euro-1"
+        if total_area < 50.0:
+            return "euro-2"
     if rooms >= 4:
         return "4+"
     if rooms in (1, 2, 3):