fix(sf-b4): price_per_m2 → price_per_m2_rub (actual column name in objective_lots)

Worker used wrong column name in KPI 3 query — would always raise
UndefinedColumn at runtime and fall back to hardcoded defaults instead
of returning real data. Verified column name via vault
Objective_Data_Source_May17.md + earlier postgres MCP query.

Also resolves merge with hotfix #337 (pilot.py EmailStr drop) — no
actual conflict, just sync with new main.

backend/app/api/v1/landing.py

@@ -54,7 +54,7 @@ def _query_stats(db: Session) -> dict[str, Any] | None:
         row_price = db.execute(
             text(
                 "SELECT"
-                "  COUNT(*) FILTER (WHERE price_per_m2 IS NOT NULL) * 100.0"
+                "  COUNT(*) FILTER (WHERE price_per_m2_rub IS NOT NULL) * 100.0"
                 "    / NULLIF(COUNT(*), 0)"
                 " FROM objective_lots"
             )