api(pulls): PR diff route materializes unbounded diffs anonymously with no cap or admission

[euxaristia]

Summary

GET /api/v1/repos/{owner}/{repo}/pulls/{n}/diff (crates/gitlawb-node/src/api/pulls.rs:136-190) runs on read_routes under optional_signature only - no brake, no permit (server.rs:342-431), with an acquire that has no timeout clamp (:150-155, unlike git_info_refs/ipfs). branch_diff (crates/gitlawb-node/src/git/store.rs:750-758) runs git diff <target>...<source> synchronously with .output() (full stdout buffered), then String::from_utf8_lossy(...).to_string() copies it again; the JSON response materializes the whole patch a third time.

Impact

Any authenticated agent can open a PR whose source branch diffs a large amount of content against a public repo (push packs reach 2 GB by default); every subsequent GET is anonymous. Each request pins a runtime worker for the subprocess duration and buffers the full diff twice, so concurrent requests stall all workers, taking /health down with them. The blocking-git-on-the-runtime aspect is #204's class; the unbounded diff serve on this route is not covered by it, and #407's ceiling covered blobs only.

Remediation

1. Cap the diff size before serving (fail closed past a ceiling) or stream with a hard cap.
2. Admit the route through a semaphore or per-IP brake like the git and ipfs groups.
3. Move the subprocess to spawn_blocking with a deadline and reaping, and clamp the acquire timeout like the smart-HTTP paths.

Proposed labels: kind:security, crate:node, subsystem:api (final severity yours).