feat(fusion_accounting_reports): PDF export with QWeb template

Adds an AbstractModel report (report_pdf.py) and a single multi-purpose
QWeb template (report_pdf_template.xml) that renders P&L, Balance Sheet,
Trial Balance, and General Ledger results from the engine.

Wires the controller's /fusion/reports/export_pdf endpoint to actually
return base64-encoded PDF bytes via _render_qweb_pdf. The template walks
the result['rows'] list and applies indentation/bold based on level and
is_subtotal flags, with optional comparison columns when present.

Tests: 2 new (test_pdf_export.py) + 1 controller test updated to assert
the real PDF response. Net 109 -> 111.

Made-with: Cursor

fusion_accounting_reports/tests/__init__.py

@@ -18,3 +18,4 @@ from . import test_pnl_integration
 from . import test_bs_tb_integration
 from . import test_account_balance_mv
 from . import test_cron
+from . import test_pdf_export