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

backend/app/services/site_finder/velocity.py

@@ -74,11 +74,15 @@ 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:
-        comp_rows = (
-            db.execute(
-                text(
-                    f"""
+        with db.begin_nested():
+            comp_rows = (
+                db.execute(
+                    text(
+                        f"""
                     WITH latest_obj AS (
                         SELECT DISTINCT ON (obj_id)
                                obj_id,
@@ -114,25 +118,20 @@ def compute_velocity(
                      ORDER BY distance_m ASC
                      LIMIT 200
                     """
-                ),
-                {
-                    "parcel_wkt": parcel_geom_wkt,
-                    "radius_m": radius_km * 1000.0,
-                    "obj_class": obj_class,
-                },
+                    ),
+                    {
+                        "parcel_wkt": parcel_geom_wkt,
+                        "radius_m": radius_km * 1000.0,
+                        "obj_class": obj_class,
+                    },
+                )
+                .mappings()
+                .all()
             )
-            .mappings()
-            .all()
-        )
     except Exception:
         logger.exception("velocity: competitor query failed for wkt=%s", parcel_geom_wkt[:80])
-        # КРИТИЧНО: rollback aborted transaction чтобы caller (analyze_parcel)
-        # мог продолжить выполнять последующие queries. Иначе caller получает
-        # cascade InFailedSqlTransaction на следующей SQL execute.
-        try:
-            db.rollback()
-        except Exception:
-            pass
+        # SAVEPOINT auto-rollbacks через __exit__ context manager.
+        # Outer tx остаётся clean — caller продолжает работать без cascade.
         return None
 
     if not comp_rows:
@@ -154,10 +153,11 @@ def compute_velocity(
     # area_sq = м² за месяц (primary). Если NULL — realised * 45 м² heuristic.
     # type = 'apartments' — только жильё.
     try:
-        sales_rows = (
-            db.execute(
-                text(
-                    """
+        with db.begin_nested():
+            sales_rows = (
+                db.execute(
+                    text(
+                        """
                     WITH latest_sg AS (
                         SELECT DISTINCT ON (obj_id, report_month)
                                obj_id,
@@ -182,22 +182,18 @@ def compute_velocity(
                      WHERE area_sq > 0 OR realised > 0
                      GROUP BY obj_id
                     """
-                ),
-                {
-                    "obj_ids": obj_ids,
-                    "window_interval": f"{months_window} months",
-                },
+                    ),
+                    {
+                        "obj_ids": obj_ids,
+                        "window_interval": f"{months_window} months",
+                    },
+                )
+                .mappings()
+                .all()
             )
-            .mappings()
-            .all()
-        )
     except Exception:
         logger.exception("velocity: sale_graph query failed for obj_ids=%s", obj_ids[:5])
-        # КРИТИЧНО: rollback aborted transaction (см. competitor query комментарий)
-        try:
-            db.rollback()
-        except Exception:
-            pass
+        # SAVEPOINT auto-rollback'нут — outer tx clean
         return None
 
     if not sales_rows:
@@ -278,10 +274,11 @@ 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:
-        row = (
-            db.execute(
-                text(
-                    """
+        with db.begin_nested():
+            row = (
+                db.execute(
+                    text(
+                        """
                     WITH latest_sg AS (
                         SELECT DISTINCT ON (obj_id, report_month)
                                obj_id,
@@ -315,19 +312,15 @@ def _get_ekb_median(db: Session, months_window: int = 6) -> float | None:
                     SELECT PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY velocity) AS median
                       FROM per_obj_velocity
                     """
-                ),
-                {"window_interval": f"{months_window} months"},
+                    ),
+                    {"window_interval": f"{months_window} months"},
+                )
+                .mappings()
+                .first()
             )
-            .mappings()
-            .first()
-        )
     except Exception:
         logger.warning("velocity: ekb_median query failed, using fallback")
-        # КРИТИЧНО: rollback aborted transaction
-        try:
-            db.rollback()
-        except Exception:
-            pass
+        # SAVEPOINT auto-rollback'нут
         return None
 
     if row and row["median"] is not None: