api(arweave): GET /arweave/anchors has no visibility gate and leaks private-repo ref names and shas

[PierrunoYT]

GET /api/v1/arweave/anchors (crates/gitlawb-node/src/api/arweave.rs:24-40, db/mod.rs:3836-3858) has no auth extractor and no visibility check. Sibling feeds (events.rs, stats, repo listings) gate every row per caller.

Impact: anonymous callers get repo, owner_did, ref_name, old_sha, new_sha and the Arweave/Irys tx id for anchors of private repos, which leaks branch names and commit tips.

Repro: GET /api/v1/arweave/anchors?repo=<owner-key>/<name> on a private repo returns 200 with ref names and shas.

Fix: apply the same per-row visibility gate events.rs uses (no existence leak), and add deny tests for an anonymous and a non-reader caller. Separate from #490 (negative limit).

Found in the Oct 2 2026 audit (A4) at bfc44f9.

[beardthelion]

Duplicate of #121, which tracks the same unauthenticated /arweave/anchors leak of private-repo ref names and shas. The fix is open at #134. Closing in favor of that one; reopen if #134 misses something this report covers.