feat(tradein): add data_quality flag to /sales-vs-listings (#721)

ADR variant 3 (human-decided): house-level ДКП↔listing join is physically
impossible (deals carry only a street centroid). Instead of null-silence at
linkage=0, expose an honest enum so the frontend can tell 'no house linkage'
(street_only) from 'no data' (no_data) from 'linked' (house_linked).

- SalesVsListingsResponse: new required field data_quality.
- _empty() → 'no_data'; main return → 'house_linked' if deals_with_listings>0
  else 'street_only' (main return guarded by 'if not rows' → total_deals>0).

Does not touch SQL linkage / cadastre / geo ingest (out of scope per ADR).

Refs #721

tradein-mvp/backend/app/api/v1/trade_in.py

@@ -1581,6 +1581,7 @@ def get_sales_vs_listings(
             deals_with_listings=0,
             linkage_rate_pct=0.0,
             median_discount_pct=None,
+            data_quality="no_data",  # #721 ADR v3: нет сделок / улица не извлеклась
             pairs=[],
         )
 
@@ -1686,5 +1687,7 @@ def get_sales_vs_listings(
         deals_with_listings=deals_with_listings,
         linkage_rate_pct=linkage_rate_pct,
         median_discount_pct=median_discount,
+        # #721 ADR v3: total_deals>0 здесь (guard `if not rows` выше → _empty).
+        data_quality="house_linked" if deals_with_listings > 0 else "street_only",
         pairs=pairs,
     )

tradein-mvp/backend/app/schemas/trade_in.py

@@ -457,6 +457,7 @@ class SalesVsListingsResponse(BaseModel):
     deals_with_listings: int  # сколько имеют связанный listing
     linkage_rate_pct: float  # deals_with_listings / total_deals * 100
     median_discount_pct: float | None  # медиана по парам с listing
+    data_quality: str  # "house_linked" | "street_only" | "no_data" (#721, ADR v3)
     pairs: list[SalesListingPair]  # все пары, sorted by deal_date DESC