fix(site-finder): cascade InFailedSqlTransaction — velocity internal rollback (#155)

* fix(site-finder): db.rollback() в velocity catches + SAVEPOINT zoning/success_rec

velocity.py 3 internal try/except возвращали None БЕЗ db.rollback(). PR #154
SAVEPOINT не помог — exception не propagates из velocity. Postgres tx
aborted внутри, cascade InFailedSqlTransaction на следующих queries.

Fix:
- velocity.py: db.rollback() в 3 catch блоках
- parcels.py: SAVEPOINT обёртки для zoning + success_recommendation

Refs: user reports 2026-05-15 cascade 500

* fix(velocity): SAVEPOINT per query (not db.rollback) — correct SQLAlchemy 2.0 pattern

Per PR #155 bot review — мой db.rollback() в catches был НЕПРАВИЛЬНЫЙ.

Session.rollback() = ENTIRE outer tx (не savepoint). Outer SAVEPOINT context
становится orphaned → InvalidRequestError на __exit__.

Correct fix: wrap каждый db.execute в свой with db.begin_nested():
- Failure → __exit__ propagates exception → SAVEPOINT rolls back
- Outer tx остаётся clean
- velocity returns None gracefully → caller продолжает

Все 3 SQL queries в velocity.py теперь SAVEPOINT-wrapped:
- compute_velocity: competitor_query
- compute_velocity: sales_rows query
- _get_ekb_median: median query

db.rollback() removed из всех 3 catches.

Refs: PR #155 bot review

---------

Co-authored-by: lekss361 <claudestars@proton.me>

backend/app/api/v1/parcels.py

@@ -1658,6 +1658,7 @@ def analyze_parcel(
         "lon": centroid_lon,
     }
     try:
+        with db.begin_nested():
             zoning_row = (
                 db.execute(
                     text("""
@@ -1692,6 +1693,7 @@ def analyze_parcel(
     success_recommendation: dict[str, Any] | None = None
     if district_row:
         try:
+            with db.begin_nested():
                 success_rows = (
                     db.execute(
                         text("""

backend/app/services/site_finder/velocity.py

@@ -74,7 +74,11 @@ def compute_velocity(
     # obj_class в domrf_kn_objects заполнен слабо (много NULL); фильтруем
     # только если явно передан.
     class_filter = "AND o.obj_class = :obj_class" if obj_class else ""
+    # SAVEPOINT per query: failure rollbacks ТОЛЬКО savepoint, не outer tx.
+    # db.rollback() здесь НЕЛЬЗЯ — он orphan'ит outer SessionTransaction
+    # (см. PR #155 bot review — SQLAlchemy 2.0 begin_nested context cleanup).
     try:
+        with db.begin_nested():
             comp_rows = (
                 db.execute(
                     text(
@@ -126,6 +130,8 @@ def compute_velocity(
             )
     except Exception:
         logger.exception("velocity: competitor query failed for wkt=%s", parcel_geom_wkt[:80])
+        # SAVEPOINT auto-rollbacks через __exit__ context manager.
+        # Outer tx остаётся clean — caller продолжает работать без cascade.
         return None
 
     if not comp_rows:
@@ -147,6 +153,7 @@ def compute_velocity(
     # area_sq = м² за месяц (primary). Если NULL — realised * 45 м² heuristic.
     # type = 'apartments' — только жильё.
     try:
+        with db.begin_nested():
             sales_rows = (
                 db.execute(
                     text(
@@ -186,6 +193,7 @@ def compute_velocity(
             )
     except Exception:
         logger.exception("velocity: sale_graph query failed for obj_ids=%s", obj_ids[:5])
+        # SAVEPOINT auto-rollback'нут — outer tx clean
         return None
 
     if not sales_rows:
@@ -266,6 +274,7 @@ def _get_ekb_median(db: Session, months_window: int = 6) -> float | None:
     Fallback к _EKB_MEDIAN_FALLBACK_SQM_PER_MONTH если нет данных в БД.
     """
     try:
+        with db.begin_nested():
             row = (
                 db.execute(
                     text(
@@ -311,6 +320,7 @@ def _get_ekb_median(db: Session, months_window: int = 6) -> float | None:
             )
     except Exception:
         logger.warning("velocity: ekb_median query failed, using fallback")
+        # SAVEPOINT auto-rollback'нут
         return None
 
     if row and row["median"] is not None: