Add Gitea attachment API fix reference

[cookesan]

Adds Gitea attachment API fix reference to GHSA-263q-5cv3-xq9g.

Audit refresh, July 8, 2026:

• Confirmed the live PR is open, ready for review, merge-clean, and the visible advisory check is green.
• Rechecked the merge-base diff: exactly one advisory JSON file changes (advisories/github-reviewed/2025/12/GHSA-263q-5cv3-xq9g/GHSA-263q-5cv3-xq9g.json), JSON parses, git diff --check passes, and the exact-head worktree is clean.
• Revalidated the added public references in the diff; all resolve through their source site or registry.

[cookesan]

Refreshed on July 8, 2026:

• GHSA-263q-5cv3-xq9g and CVE-2025-68939 remain public and consistent with this advisory.
• Verified go-gitea/gitea PR #32151, commit 7adc4717ec8e4f8fe678010866e936cf024f498d, release v1.23.0, and the Go module proxy record for code.gitea.io/gitea v1.23.0.
• Local validation passed: jq empty and git diff --check.

No affected range, package, or severity changes are included.