fix(tradein-estimator): restore CAST in IS NOT NULL predicates (revert audit #9)

PR #509 removed CAST(:target_year AS integer) and CAST(:target_house_type AS text)
from the IS NOT NULL predicates in _fetch_analogs, citing audit finding #9 (perf).
This broke production: psycopg3 prepared statement could not determine $4 type
when target_house_type=None, raising AmbiguousParameter at plan time. The CAST
in the THEN branch is not enough — PG planner needs the type to evaluate
IS NOT NULL itself.

The inline comment at lines 814-816 explicitly documented this requirement;
PR #509 misread the audit finding. Per-row CAST cost on ≤300 rows is negligible
vs production breakage. Restoring CAST forms; THEN-branch CASTs untouched
(they were never removed).

Verified prod stack trace 2026-05-24 ~12:25 (target_year=1978, target_house_type=None).

tradein-mvp/backend/app/services/estimator.py

@@ -881,13 +881,13 @@ def _fetch_analogs(
                         -- без типа → PostgreSQL "could not determine data type of parameter"
                         -- (AmbiguousParameter). Явный тип снимает неоднозначность.
                         + CASE
-                            WHEN :target_year IS NOT NULL
+                            WHEN CAST(:target_year AS integer) IS NOT NULL
                                  AND year_built IS NOT NULL
                             THEN abs(year_built - CAST(:target_year AS integer)) / 12.0
                             ELSE 0
                           END
                         + CASE
-                            WHEN :target_house_type IS NOT NULL
+                            WHEN CAST(:target_house_type AS text) IS NOT NULL
                                  AND house_type IS NOT NULL
                                  AND house_type <> CAST(:target_house_type AS text)
                             THEN 1.5
@@ -900,13 +900,13 @@ def _fetch_analogs(
                             ST_Distance(geom::geography, ST_MakePoint(:lon, :lat)::geography)
                                 / 1000.0
                             + CASE
-                                WHEN :target_year IS NOT NULL
+                                WHEN CAST(:target_year AS integer) IS NOT NULL
                                      AND year_built IS NOT NULL
                                 THEN abs(year_built - CAST(:target_year AS integer)) / 12.0
                                 ELSE 0
                               END
                             + CASE
-                                WHEN :target_house_type IS NOT NULL
+                                WHEN CAST(:target_house_type AS text) IS NOT NULL
                                      AND house_type IS NOT NULL
                                      AND house_type <> CAST(:target_house_type AS text)
                                 THEN 1.5