SAN DIEGO — San Diego is suing the Grand Del Mar luxury hotel complex in Carmel Valley for nearly $1.5 million in unpaid fees that are allegedly required by the project’s development agreement and a ballot measure that allowed its construction. The lawsuit, filed April 21, says the city treasurer’s office tried unsuccessfully to collect the money before seeking a legal remedy. The Newport Beach law firm listed as representing the hotel, the Sitzer Law Group, did not respond Tuesday to requests for comment. The bulk of the allegedly unpaid fees, $1.1 million, came due Dec. 30 — during a pandemic that has created significant financial challenges for nearly every business in San Diego’s tourism industry.