Add POI and POIDistance data models with migration

Introduces PointOfInterest (per-user named locations with lat/lng) and
POIDistance (travel time/distance per listing+POI+mode triple) SQLModel
entities, plus an Alembic migration to create both tables with indexes
and a composite unique constraint.

models/poi.py

@@ -0,0 +1,13 @@
+from datetime import datetime
+
+from sqlmodel import SQLModel, Field
+
+
+class PointOfInterest(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    user_id: int = Field(foreign_key="user.id", index=True)
+    name: str = Field(nullable=False)
+    address: str = Field(nullable=False)
+    latitude: float = Field(nullable=False)
+    longitude: float = Field(nullable=False)
+    created_at: datetime = Field(default_factory=datetime.utcnow)

models/poi_distance.py

@@ -0,0 +1,23 @@
+from datetime import datetime
+
+from sqlmodel import SQLModel, Field, UniqueConstraint
+
+from models.listing import ListingType
+
+
+class POIDistance(SQLModel, table=True):
+    __table_args__ = (
+        UniqueConstraint(
+            "listing_id", "listing_type", "poi_id", "travel_mode",
+            name="uq_poi_distance_listing_poi_mode",
+        ),
+    )
+
+    id: int | None = Field(default=None, primary_key=True)
+    listing_id: int = Field(index=True, nullable=False)
+    listing_type: ListingType = Field(index=True, nullable=False)
+    poi_id: int = Field(foreign_key="pointofinterest.id", index=True, nullable=False)
+    travel_mode: str = Field(nullable=False)  # WALK, BICYCLE, TRANSIT
+    duration_seconds: int = Field(nullable=False)
+    distance_meters: int = Field(nullable=False)
+    computed_at: datetime = Field(default_factory=datetime.utcnow)