feat(fusion_accounting_l10n_ca): Canadian reports + tax return tracking

Replaces Enterprise's l10n_ca_reports with Fusion-native equivalents:
- ca_balance_sheet, ca_profit_loss as fusion.report definitions
- fusion.tax.return model for GST/HST/PST/T4/T5018 filing tracking
- Auto-installs when l10n_ca + fusion_accounting_reports both present

Removes Westin's last Canadian-compliance dependency on Enterprise's
account_reports.

Made-with: Cursor

fusion_accounting_l10n_ca/models/__init__.py

@@ -0,0 +1 @@
+from . import fusion_tax_return