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
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