Add POI API routes and Celery task

FastAPI router with CRUD endpoints for POIs, distance calculation
trigger, and distance queries. Streaming GeoJSON endpoint now accepts
include_poi_distances=true to inject travel times into features.
Celery task wraps the distance calculator with progress reporting.

celery_app.py

@@ -9,7 +9,7 @@ app = Celery(
     "celery_app",
     broker=os.getenv("CELERY_BROKER_URL", "redis://localhost:6379/0"),
     backend=os.getenv("CELERY_RESULT_BACKEND", "redis://localhost:6379/1"),
-    include=["tasks.listing_tasks"],
+    include=["tasks.listing_tasks", "tasks.poi_tasks"],
 )
 
 app.conf.update(