Add Stripe fee tracking on paid invoices + backfill function

- Store stripe_fee on invoices when webhook receives checkout.session.completed
- Display Stripe fee and net received in InvoiceDetail when paid via Stripe
- Add backfill-stripe-fees edge function to populate fee on existing paid invoices
- Migration: add stripe_fee column to invoices table
- Includes all pending portal changes (brand book, sign survey, task/project/company updates, etc.)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

supabase/migrations/20260414120000_add_stripe_fee_to_invoices.sql

@@ -0,0 +1,2 @@
+-- Add stripe_fee column to store the Stripe processing fee on paid invoices
+alter table public.invoices add column if not exists stripe_fee numeric(10,2);