Persistent definition of a Fusion financial report. Each report (P&L,
balance sheet, trial balance, GL) has one row in fusion.report holding
its metadata + line specs (stored as JSON for layout flexibility).
V19 conventions: models.Constraint inline, no _sql_constraints. Per-
company uniqueness on (company_id, code).
3 new tests, 27 total passing.
Made-with: Cursor
