feat(v5.2.0): finalize Infrastructure & Polish (tests isolation, xdist, lint, CI)

tests/conftest.py: mutate the settings singleton (instead of rebinding) so DB + backup dir are isolated per test -> pytest-xdist safe.
Real backup tests (snapshot/prune/admin API) and OAuth mock tests (Gitea/GitHub/link) replace the previous skips.
init_db() now also creates webhook_subscriptions (full schema without the FastAPI lifespan).
ruff check is clean; .eslintrc.json migrated to eslint.config.mjs (flat config).
CI: lint job (ruff + eslint), parallel tests (-n auto), run on every branch push.
VERSION 5.11.1.

app/routers/export.py

@@ -74,10 +74,10 @@ async def export_pdf(page_id: int, request: Request):
     try:
         pdf_bytes = page_to_pdf_bytes(page)
     except ImportError:
-        raise HTTPException(status_code=501, detail="PDF export requires 'weasyprint' or 'xhtml2pdf'")
+        raise HTTPException(status_code=501, detail="PDF export requires 'weasyprint' or 'xhtml2pdf'") from None
     except Exception as exc:  # noqa: BLE001
         logger.error("PDF export failed for page %s: %s", page_id, exc)
-        raise HTTPException(status_code=500, detail="PDF generation failed")
+        raise HTTPException(status_code=500, detail="PDF generation failed") from exc
     filename = _safe_filename(page, "pdf")
     headers = _download_header(filename, "application/pdf")
     return Response(content=pdf_bytes, status_code=200, headers=headers)