feat: Update endpoint countries

app/api/endpoints/countries.py

@@ -0,0 +1,7 @@
+@router.get("/countries", response_model=List[CountryResponse])
+async def get_countries(db: Session = Depends(get_db)):
+    """
+    Fetch the list of countries from the database.
+    """
+    countries = db.query(Country).all()
+    return [CountryResponse.from_orm(country) for country in countries]