fix(sf-16): derive price_per_m2 from price_rub in _norm_flat + backfill SQL

Audit confirmed: DOM.RF /portal/table API returns price=null for 99.7% of flats
(both price_rub and price_per_m2 are NULL). Root cause is API limitation — per-flat
pricing lives in a separate catalog endpoint not yet scraped (BUG #22).

Adds defensive fallback in _norm_flat(): when price_rub is present but
pricePerSquareMeter is absent, derive price_per_m2 = round(price_rub / total_area, 2).
Covers partial API responses from any future endpoint that omits the computed rate.

Adds 107_backfill_price_per_m2.sql for historical rows (~849 affected).

backend/app/services/scrapers/domrf_kn.py

@@ -262,6 +262,11 @@ def _norm_flat(row: dict[str, Any], region_cd: int | None) -> dict[str, Any]:
     Real fields: flatId|id (numeric, may be null), odsId, elemId (uuid hash),
     type, number, isStudio, totalArea, livingArea, rooms, status (free|booked|sold),
     price, pricePerSquareMeter, numberFloors. Plus injected _objId and _floor.
+
+    Note: /portal/table returns price=null for most objects (sold/booked flats and
+    objects that don't expose per-flat pricing via this endpoint). price_per_m2 is
+    derived from price_rub / total_area when the API returns price_rub but omits
+    pricePerSquareMeter — defensive fallback for partial API responses.
     """
     flat_id = _g(row, "flatId", "id")
     if flat_id is None:
@@ -269,18 +274,34 @@ def _norm_flat(row: dict[str, Any], region_cd: int | None) -> dict[str, Any]:
         elem = _g(row, "elemId")
         if elem:
             flat_id = abs(hash(elem)) % (2**63 - 1)
+
+    price_rub = _g(row, "price")
+    price_per_m2 = _g(row, "pricePerSquareMeter")
+    total_area = _g(row, "totalArea")
+
+    # Derive price_per_m2 when API returns price_rub but omits pricePerSquareMeter.
+    # Covers cases where the table endpoint has the flat price but no pre-computed m² rate.
+    if price_per_m2 is None and price_rub is not None and total_area and total_area > 0:
+        price_per_m2 = round(price_rub / total_area, 2)
+        logger.info(
+            "derive price_per_m2=%.2f for flat ods_id=%s obj_id=%s",
+            price_per_m2,
+            _g(row, "odsId"),
+            _g(row, "_objId", "objId"),
+        )
+
     return {
         "id": flat_id,
         "ods_id": _g(row, "odsId"),
         "flat_type": _g(row, "type", "flatType"),
         "flat_number": _g(row, "number", "flatNumber"),
         "is_studio": _to_bool(_g(row, "isStudio")),
-        "total_area": _g(row, "totalArea"),
+        "total_area": total_area,
         "living_area": _g(row, "livingArea"),
         "rooms": _g(row, "rooms"),
         "status": _g(row, "status"),
-        "price_rub": _g(row, "price"),
+        "price_rub": price_rub,
-        "price_per_m2": _g(row, "pricePerSquareMeter"),
+        "price_per_m2": price_per_m2,
         "floor": _g(row, "_floor", "floor"),
         "num_floors": _g(row, "numberFloors"),
         "obj_id": _g(row, "_objId", "objId"),

data/sql/107_backfill_price_per_m2.sql

@@ -0,0 +1,24 @@
+-- 107_backfill_price_per_m2.sql
+-- Backfill price_per_m2 = ROUND(price_rub / total_area, 2) for rows where
+-- price_rub is known but price_per_m2 was not stored by the scraper.
+--
+-- Context (SF FixList #16):
+--   domrf_kn.py _norm_flat() now derives price_per_m2 on ingest.
+--   This one-time UPDATE covers historical rows scraped before the fix.
+--   For Малевич (obj_id=64701) and the wider dataset, both price_rub AND
+--   price_per_m2 are NULL (DOM.RF /portal/table API does not return per-flat
+--   pricing for most objects). Those rows are unaffected by this UPDATE.
+--   Only rows where price_rub IS NOT NULL benefit — currently ~849 rows.
+--
+-- Safe to re-run: idempotent (WHERE price_per_m2 IS NULL).
+
+BEGIN;
+
+UPDATE domrf_kn_flats
+SET price_per_m2 = ROUND((price_rub / total_area)::numeric, 2)
+WHERE price_per_m2 IS NULL
+  AND price_rub IS NOT NULL
+  AND total_area IS NOT NULL
+  AND total_area > 0;
+
+COMMIT;