Add backend/app/services/exporters/report_pdf.py: export_report_pdf(report)->bytes,
the additive PDF counterpart to excel.export_report_xlsx (#991). Renders a
SiteFinderReport (#987) to PDF via WeasyPrint across the 6 §13 content sections
(Сводка/Рынок сейчас/Будущий рынок/Продукт ТЗ/Сценарии/Скоринг).
Mirrors house style: _normalize (instance or as_dict), named-const titles/CSS/
microcopy, RU labels, None→"—", graceful-never-raise, ADVISORY marker, local
weasyprint import. All dynamic fields html.escape'd. STRICTLY ADDITIVE — zero
edits to the 3 live PDF exporters (tradein/layout/snapshot) per tech-analyst
blast-radius analysis; "unify pdf.py" deferred (would rewrite 3 working features
for no gain). Endpoint wiring is #959. 16 tests (render proven via Rosetta x86_64;
6 native-render cases need Docker/CI matched-arch libgobject — pure-logic green).
