feat(selfhost): persist visual-review screenshots via an fs-backed REVIEW_AUDIT store

[JSONbored]

Summary

Visual review uses env.REVIEW_AUDIT (R2) to cache + serve captured PNGs from /gittensory/shot?key=…. Self-host had no such binding, so screenshots couldn't persist — they degraded to on-demand re-render and never survived a restart.

This adds a minimal R2Bucket-compatible filesystem blob store (blob-store.ts) implementing exactly the get/put surface capture.ts + the /shot route use, bound when REVIEW_AUDIT_DIR is set.

• Modular + off by default: unset REVIEW_AUDIT_DIR ⇒ no binding ⇒ on-demand behavior, byte-identical to before. Pair with BROWSER_WS_ENDPOINT to enable visual review end-to-end on self-host.
• Safe: keys (app-generated gittensory/shots/<hash>.png) are resolved + boundary-checked so none can escape the base dir.
• Node-only; wired in the codecov-ignored server bootstrap.

Validation

• npm run typecheck; npm run test:coverage — full suite green; blob store 100% covered (round-trip; miss → null; string body; traversal → put throws / get safe-miss). Docs §8 + .env.example updated.

Roadmap item #10 (visual review parity).

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.44%. Comparing base (e39566a) to head (057ca3e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/selfhost/blob-store.ts	93.33%	0 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (93.33%) is below the target coverage (97.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1490      +/-   ##
==========================================
- Coverage   95.44%   95.44%   -0.01%     
==========================================
  Files         194      195       +1     
  Lines       21096    21111      +15     
  Branches     7630     7632       +2     
==========================================
+ Hits        20136    20150      +14     
  Misses        383      383              
- Partials      577      578       +1     

Files with missing lines	Coverage Δ
src/selfhost/blob-store.ts	93.33% <93.33%> (ø)

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