feat(fusion_accounting_reports): fusion.report definition model

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

fusion_accounting_reports/__init__.py

@@ -1 +1,2 @@
 from . import services
+from . import models