Support modifier-driven flat rate (piece) pay by sqft, equipment count, and units

Top Choice (and similar acquisitions) need piece-rate pay that varies by job attributes, not by price (for regulatory reasons). Examples: home inspection $50 base + $25 per 1k sq ft over 4k; mold $40 for first four samples + $10 per additional; multifamily $25/unit + hourly. Today modifiers only affect cost, time, equipment, and admin – not flat rate pay. We need to add a “flat rate” modifier type that can use the same bases (e.g. sqft, equipment count, unit count) and have that feed into payroll so inspector pay is calculated correctly for piece work.

Benefits: Unblocks Top Choice launch and supports the acquisition model; other companies (e.g. Southeast, AJF) have had similar needs. Reusable for future piece-rate pay structures.