This will install the file in user-selected location, ie. same drive as the app binaries. Note however that contents under \Data are visible also in the File Manager so users may (more or less inadvertently) delete or otherwise tamper with the file.
database.pkg_postrules = "\"mydb.db\" - \"!:\\Data\\mydb.db\"
DEPLOYMENT += database
Also, if you want your application to survive a backup & restore process, you'll need something like this:
As for the contents of backup_registration.xml, check TSS000750. Note that if you use /Data as the location, you don't need to specify a public_backup section as the folder is always backed up as user data.
backupxml.sources = "backup_registration.xml"
DEPLOYMENT += backupxml