Legal Question in Personal Injury in California

Yes. You can be sued. You can raise whatever valid defense you may have. Turn it over to your homeowners insurance if you have that.

Dog bites involve strict liability even if it's the first time, (there is no first bite right defense). It is not based on negligence. Advise your homeowners insurance company of all the facts. Good luck.

Yes, it is assumed you want delivery persons to come onto your property so no trespassing defense and the biscuit is used to make angry dogs friendly but your dog didn't like the biscuit. By law you are required to protect people you allow onto your property or the law allows to come onto your property such as delivery people, selling people and emergency personnel. You could have posted a large don't trespass-bad dog sign and then no one would come onto your property except the police in case of an emergency who would shoot your dog first and ask questions later. So people who have dogs and want to get mail delivered and emergency services need to carry insurance or keep the dog in an area where no one can be expected to go unless there to burglarize the house. Liability insurance, including coverage for dog bites, is included in homeowners insurance or renters insurance. Hopefully you have coverage and, if you have it, you are required by the policy to let the insurance company handle the claim.