Fix POI distance calculation reliability for remote/Celery execution

- Fix silent log loss: replace hardcoded "uvicorn.error" logger with __name__
  in osrm_client, otp_client, poi_distance_calculator, and poi_tasks (uvicorn
  logger has no handlers in Celery worker, so all errors were silently dropped)
- Add Celery retry: autoretry_for=(Exception,), max_retries=3, retry_backoff
- Add top-level exception handling in task with full traceback logging
- Fix upsert_distances: replace session.merge() (PK-based) with proper
  dialect-aware INSERT ON DUPLICATE KEY UPDATE / ON CONFLICT DO UPDATE
- Filter out listings with null/zero coordinates before routing
- Raise OSError when all routing engines fail with 0 results computed,
  distinguishing "nothing to compute" from "all engines unreachable"

repositories/poi_repository.py

@@ -52,9 +52,41 @@ class POIRepository:
             return True
 
     def upsert_distances(self, distances: list[POIDistance]) -> None:
+        """Insert or update POI distances, handling duplicate unique constraints."""
+        if not distances:
+            return
         with Session(self.engine) as session:
+            dialect = self.engine.dialect.name
             for dist in distances:
-                session.merge(dist)
+                values = {
+                    "listing_id": dist.listing_id,
+                    "listing_type": dist.listing_type,
+                    "poi_id": dist.poi_id,
+                    "travel_mode": dist.travel_mode,
+                    "duration_seconds": dist.duration_seconds,
+                    "distance_meters": dist.distance_meters,
+                    "computed_at": dist.computed_at,
+                }
+                if dialect == "mysql":
+                    from sqlalchemy.dialects.mysql import insert as mysql_insert
+                    stmt = mysql_insert(POIDistance).values(**values)
+                    stmt = stmt.on_duplicate_key_update(
+                        duration_seconds=stmt.inserted.duration_seconds,
+                        distance_meters=stmt.inserted.distance_meters,
+                        computed_at=stmt.inserted.computed_at,
+                    )
+                else:
+                    from sqlalchemy.dialects.sqlite import insert as sqlite_insert
+                    stmt = sqlite_insert(POIDistance).values(**values)
+                    stmt = stmt.on_conflict_do_update(
+                        index_elements=["listing_id", "listing_type", "poi_id", "travel_mode"],
+                        set_={
+                            "duration_seconds": stmt.excluded.duration_seconds,
+                            "distance_meters": stmt.excluded.distance_meters,
+                            "computed_at": stmt.excluded.computed_at,
+                        },
+                    )
+                session.execute(stmt)
             session.commit()
 
     def get_distances_for_listings(