fix(pzz_loader): replace bare db.rollback() with SAVEPOINT per-row (#124)

Per code review audit (May 14): transaction correctness bug в
`sync_pzz_zones_to_db()` — inner `db.rollback()` в loop сбрасывал outer
transaction на per-row failures → counts inserted/updated НЕ matched DB state.

## Fix

Wrap UPSERT в loop через `db.begin_nested()` (SAVEPOINT):
- Failure одной row → rollback только её savepoint, outer tx alive
- Successfully inserted rows accumulate в outer tx → commit final
- Outer try/except добавлен для unexpected error → rollback + log + raise
- `finally: db.close()` preserved

Pattern consistent с `domrf_kn.py` (где SAVEPOINT уже работает).

## Preserved

- Function signature + return shape {fetched, inserted, updated, skipped} — unchanged
- Logging contracts
- skipped count теперь корректно включает DB-failure rows (consistent with prior behavior)

## Tests

- No test_pzz_loader.py — skipped
- ruff + AST passed

## Vault

`fixes/Bug_Pzz_Loader_Missing_Savepoint_May14.md` — created (status: resolved).

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

backend/app/services/site_finder/pzz_loader.py

@@ -80,6 +80,10 @@ def sync_pzz_zones_to_db() -> dict[str, int]:
     """Загрузить зоны из PKK6 и UPSERT в pzz_zones_ekb.
 
     Возвращает словарь {fetched, inserted, updated, skipped}.
+
+    Каждый UPSERT обёрнут в SAVEPOINT (db.begin_nested), чтобы ошибка
+    одной записи не откатывала всю транзакцию и счётчики оставались
+    согласованными с тем, что реально записано в DB.
     """
     features = asyncio.run(fetch_pkk6_zones())
     inserted = 0
@@ -105,41 +109,45 @@ def sync_pzz_zones_to_db() -> dict[str, int]:
             )
             description = props.get("description")
             try:
-                result = db.execute(
-                    text("""
-                        INSERT INTO pzz_zones_ekb
-                            (rosreestr_id, zone_code, zone_name, description, raw_props, geom)
-                        VALUES (
-                            :rid, :code, :nm, :desc, CAST(:props AS jsonb),
-                            ST_Multi(ST_SetSRID(ST_GeomFromGeoJSON(:g), 4326))
-                        )
-                        ON CONFLICT (rosreestr_id) DO UPDATE
-                           SET zone_code   = EXCLUDED.zone_code,
-                               zone_name   = EXCLUDED.zone_name,
-                               description = EXCLUDED.description,
-                               raw_props   = EXCLUDED.raw_props,
-                               geom        = EXCLUDED.geom,
-                               fetched_at  = NOW()
-                        RETURNING (xmax = 0) AS is_insert
-                    """),
-                    {
-                        "rid": rosreestr_id,
-                        "code": zone_code,
-                        "nm": zone_name,
-                        "desc": description,
-                        "props": json.dumps(props, ensure_ascii=False),
-                        "g": json.dumps(geom),
-                    },
-                ).scalar()
+                with db.begin_nested():  # SAVEPOINT — откат только этой записи
+                    result = db.execute(
+                        text("""
+                            INSERT INTO pzz_zones_ekb
+                                (rosreestr_id, zone_code, zone_name, description, raw_props, geom)
+                            VALUES (
+                                :rid, :code, :nm, :desc, CAST(:props AS jsonb),
+                                ST_Multi(ST_SetSRID(ST_GeomFromGeoJSON(:g), 4326))
+                            )
+                            ON CONFLICT (rosreestr_id) DO UPDATE
+                               SET zone_code   = EXCLUDED.zone_code,
+                                   zone_name   = EXCLUDED.zone_name,
+                                   description = EXCLUDED.description,
+                                   raw_props   = EXCLUDED.raw_props,
+                                   geom        = EXCLUDED.geom,
+                                   fetched_at  = NOW()
+                            RETURNING (xmax = 0) AS is_insert
+                        """),
+                        {
+                            "rid": rosreestr_id,
+                            "code": zone_code,
+                            "nm": zone_name,
+                            "desc": description,
+                            "props": json.dumps(props, ensure_ascii=False),
+                            "g": json.dumps(geom),
+                        },
+                    ).scalar()
                 if result:
                     inserted += 1
                 else:
                     updated += 1
             except Exception as e:
                 logger.warning("PZZ insert failed for rosreestr_id=%s: %s", rosreestr_id, e)
-                db.rollback()
                 skipped += 1
         db.commit()
+    except Exception as e:
+        db.rollback()
+        logger.exception("pzz_loader: unexpected error, outer tx rolled back: %s", e)
+        raise
     finally:
         db.close()
     return {