fix: sanitize PDF export filenames against path traversal

[tsumon]

Summary

Markdown export already sanitized the IR topic before writing a file. PDF export did not.

CLI export_pdf.py interpolated metadata.topic into final_reports/pdf/report_{topic}_{timestamp}.pdf. A topic such as ../../tmp/pwned writes outside the export directory.

The HTTP routes /api/report/export/pdf/<task_id> and /export/pdf-from-ir put the same raw topic into Content-Disposition: attachment; filename="...". CRLF or quotes in the topic become response-header injection.

This PR:

• Adds ReportEngine/utils/filenames.py with a shared sanitizer (letters, digits, space, -, _; spaces become _).
• Uses it for Markdown export, both PDF HTTP routes, and the CLI exporter.
• Resolves the CLI output path and refuses to write outside final_reports/pdf.
• Treats missing / non-dict metadata as a fallback (task.query or "report") instead of crashing on .get.

No Pango/WeasyPrint required for the tests; PDF rendering is stubbed.

How to verify

python -m pytest tests/test_filenames.py tests/test_export_pdf_paths.py tests/test_export_pdf_headers.py tests/test_report_cancel_route.py -q

18 passed.

[666ghj]

Agent automated reply on behalf of the maintainer.

Thank you for this focused export improvement. Sharing filename handling between the export paths and checking the CLI output directory are useful changes.

Our read-only review found a fallback issue that needs correction before merge. sanitize_filename_segment() returns fallback unchanged when the primary value becomes empty, and the HTTP routes pass the original task.query as that fallback. For example, a topic of /// and a query of A/B comparison put a slash back into the final filename. This also regresses the existing Markdown helper, which fell back to the fixed value report.

Please sanitize the fallback too, then use a fixed safe default if both values are empty. Add regression cases for an empty sanitized topic combined with a query containing slashes, quotes, or line breaks, including a Markdown export route test.

Please also distinguish filename and response-header validation from proven header injection in the description: Werkzeug rejects header values containing CR or LF. The renderer stubs are appropriate for focused filename tests, but do not establish real PDF rendering compatibility.

We have not run this PR's tests in this pass. We will keep the PR open for the focused fixes and subsequent local validation.