fix(site-finder): compute nearest_top3 metro from osm_poi_ekb (#1667)

Replace bare metro_block placeholder ({"nearest_top3": None}, blocked on a
never-merged 22h metro scraper) with a direct KNN query against osm_poi_ekb
category='metro_stop' (loaded by poi_loader). Returns 3 nearest stations with
name + rounded distance_m; empty list when none nearby (vs None on error),
SAVEPOINT + try/except like adjacent SF-B5 blocks. No frontend consumer yet.

Closes #1667

backend/app/api/v1/parcels.py

@@ -2127,8 +2127,44 @@ def analyze_parcel(
     except Exception as e:
         logger.warning("red_lines_block query failed for %s: %s", cad_num, e)
 
-    # B5-4) Metro placeholder — заполнится после merge 22h metro scraper
-    metro_block: dict[str, Any] = {"nearest_top3": None}
+    # B5-4) Metro — ближайшие 3 станции метро к участку.
+    # Данные уже в osm_poi_ekb (category='metro_stop', грузятся poi_loader.py:44),
+    # скрапер не нужен — прямой KNN-запрос по geom <-> centroid участка.
+    # nearest_top3=[] (НЕ None) когда метро не найдено: отличаем "посчитано, рядом
+    # нет" от "не реализовано". None только при ошибке запроса.
+    metro_block: dict[str, Any] = {"nearest_top3": []}
+    try:
+        with db.begin_nested():
+            metro_rows = (
+                db.execute(
+                    text("""
+                    SELECT name,
+                           ST_Distance(
+                               m.geom::geography,
+                               ST_Centroid(ST_GeomFromText(:wkt, 4326))::geography
+                           ) AS dist_m
+                      FROM osm_poi_ekb m
+                     WHERE m.category = 'metro_stop'
+                     ORDER BY m.geom <-> ST_Centroid(ST_GeomFromText(:wkt, 4326))
+                     LIMIT 3
+                """),
+                    {"wkt": geom_wkt},
+                )
+                .mappings()
+                .all()
+            )
+        metro_block = {
+            "nearest_top3": [
+                {
+                    "name": mr["name"],
+                    "distance_m": round(float(mr["dist_m"])) if mr["dist_m"] is not None else None,
+                }
+                for mr in metro_rows
+            ]
+        }
+    except Exception as e:
+        logger.warning("metro_block query failed for %s: %s", cad_num, e)
+        metro_block = {"nearest_top3": None}
 
     # B5-5) District price ranges из objective_lots (SF-B5)
     district_price_block: dict[str, Any] = {