Creates a new adder for a specific project.

Business Context:
Adders represent additional upgrades, equipment, or work items beyond the base solar
installation. Use this endpoint to add upgrades such as electrical work, structural
improvements, additional equipment, or site preparation items to a project.

Common Adder Types:

• Electrical Upgrades: Panel upgrades, sub-panels, electrical work
• Structural Work: Roof repairs, reinforcement, re-roofing
• Additional Equipment: EV chargers, energy storage, monitoring systems
• Site Preparation: Tree removal, trenching, access improvements
• Permits and Fees: Additional permit costs, utility fees

Business Rules:

• Project must be in "new" status to add adders
• Adders can only be added to projects you have permission to modify
• Cost calculations may trigger project total updates
• Customer approval may be required for significant additions

Common Use Cases:

• Adding customer-requested upgrades during proposal phase
• Including necessary electrical work identified during site survey
• Adding equipment discovered during engineering review
• Including permit fees and additional costs