Budget Workbook: editable unit count for per-unit assessment rate

The per-unit assessment (annual expenses / 12 / units) used the live association
unit count only. Add an override so the rate can use weighted/excluded units,
persisted on accounting.budget_workbooks.unit_override (null = live count). New
"# Units" control with a reset link; summary card and CSV use the effective count.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

supabase/migrations/20260609120000_budget_workbook_unit_override.sql

@@ -0,0 +1,5 @@
+-- Budget Workbook: let the per-unit assessment rate use an overridden unit count
+-- (e.g. weighted units, excluding developer-owned). Null = use the live count of
+-- units in the association.
+alter table accounting.budget_workbooks
+  add column if not exists unit_override integer;