Add missing lock_database_exclusive() to initiate_upload()
Fix a concurrency bug where initiation of two image uploads to the same repo at the same time could result in
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) database is locked [SQL: INSERT INTO upload (created, repo_id) VALUES (?, ?)] [parameters: ('2023-01-12 20:30:56.946049', 4)]