fix(uploads): cap attachment size and refuse browser-executable MIME types

[jamofer]

Summary

Attachment endpoints accepted any file of any size: shutil.copyfileobj(file.file, dest) streamed straight to disk with no upper bound and no content-type validation. Two real failure modes:

• Disk-fill DoS: any authenticated user could push gigabytes per request. With no cap the only limit was uvicorn's request body timeout.
• Stored XSS / drive-by: an HTML or SVG-with-script attachment served back through the download endpoint renders inline in the victim's browser. The download handler does set a filename header but the browser still honours Content-Type: text/html for inline rendering, so the attack worked end to end.

Fix:

• Two new env-tunable settings in core/config.py:
  • MAX_ATTACHMENT_BYTES (default 25 MiB) — generous enough for screenshots, junit bundles, short screen recordings; well below disk-fill territory.
  • BLOCKED_ATTACHMENT_MIME_TYPES (CSV) — defaults block text/html, application/xhtml+xml, application/javascript, text/javascript, image/svg+xml, application/x-msdownload, application/x-httpd-php, text/x-php. Parsed on demand via a blocked_attachment_mime_types property so admins can override per deployment.
• New services/uploads.read_attachment(file) validates the MIME (415 on hit) then drains bytes through the existing read_bounded helper (413 on overflow). Returns the bytes for the caller to persist.
• Wired into all five attachment handlers — bug, test case, execution, result, and version — each replacing the old shutil.copyfileobj + os.path.getsize pattern with payload = uploads.read_attachment(file) + len(payload).

Tests (backend/tests/security/test_upload_caps.py, parametrized × 5 endpoints):

• Oversize payload above a monkeypatched cap returns 413 on every endpoint.
• text/html body returns 415 with the offending MIME echoed in the detail.
• image/svg+xml body returns 415 (covers the script-bearing SVG vector specifically).
• Allowed text/plain payload still succeeds with 201.

Full backend suite: 894 passing.