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"

tasks/poi_tasks.py

@@ -11,7 +11,7 @@ from repositories.listing_repository import ListingRepository
 from repositories.poi_repository import POIRepository
 from services.poi_distance_calculator import calculate_poi_distances
 
-logger = logging.getLogger("uvicorn.error")
+logger = logging.getLogger(__name__)
 
 celery_logger = logging.getLogger("celery.task")
 if not celery_logger.handlers:
@@ -23,7 +23,13 @@ if not celery_logger.handlers:
     celery_logger.setLevel(logging.INFO)
 
 
-@app.task(bind=True)
+@app.task(
+    bind=True,
+    autoretry_for=(Exception,),
+    max_retries=3,
+    retry_backoff=True,
+    retry_backoff_max=300,
+)
 def calculate_poi_distances_task(
     self: Task,
     poi_id: int,
@@ -70,17 +76,23 @@ def calculate_poi_distances_task(
             "message": message,
         })
 
-    total = asyncio.run(
-        calculate_poi_distances(
-            listing_repo=listing_repo,
-            poi_repo=poi_repo,
-            poi=poi,
-            travel_modes=travel_modes,
-            listing_type=lt,
-            listing_ids=listing_ids,
-            on_progress=on_progress,
+    try:
+        total = asyncio.run(
+            calculate_poi_distances(
+                listing_repo=listing_repo,
+                poi_repo=poi_repo,
+                poi=poi,
+                travel_modes=travel_modes,
+                listing_type=lt,
+                listing_ids=listing_ids,
+                on_progress=on_progress,
+            )
         )
-    )
+    except Exception:
+        celery_logger.exception(
+            f"POI distance calculation failed: poi_id={poi_id}"
+        )
+        raise  # Let Celery's autoretry handle it
 
     celery_logger.info(f"POI distance calculation complete: {total} distances computed")