perf(review): cache the AI review by (repo, pr, head SHA) to skip redundant LLM calls

[JSONbored]

Summary

The single biggest self-host compute win: cache the AI review by (repo, pull, head SHA). On self-host
there's no Cloudflare AI Gateway, so every re-trigger re-spends the full LLM review. In particular, in
block mode the ~2-minute re-gate sweep re-runs the AI for every open PR (reReviewStoredPullRequest passes
skipAiReview: false in block mode) — re-reviewing identical code on a loop.

The expensive runAiReviewForAdvisory is now wrapped at its single call site with a commit-keyed cache:

• Hit (same repo+pr+head SHA, same review mode) → reuse the stored {notes, reviewerCount}, no LLM call.
• Miss → run the review, then store it.
• Invalidation: a new head SHA (new code) is a new key; a changed review mode (block↔advisory materially
  changes the prompt) is treated as a miss. The deterministic gate still re-evaluates every time — only the
  AI leg is reused, so CI/mergeability changes are never served stale.

getCachedAiReview / putCachedAiReview (repositories.ts) handle the nullish-head-SHA
case internally (so the call site has no dead branches), backed by a new ai_review_cache table (migration 0074).

Scope

• src/** + migration + tests; the deterministic gate path is unchanged.
• No secrets; parameterized SQL only.

Validation

• npm run typecheck, npm run db:migrations:check (contiguous 0001..0074)
• npm run test:coverage — full suite green; 100% changed lines + branches. Unit tests cover the SHA/mode/nullish matrix; an integration test proves the block-mode sweep reuses the cache with zero AI calls.

Roadmap item #1 (highest-value perf win).

[superagent-security]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.44%. Comparing base (8309b23) to head (c7ea780).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1462   +/-   ##
=======================================
  Coverage   95.44%   95.44%           
=======================================
  Files         194      194           
  Lines       21054    21064   +10     
  Branches     7619     7623    +4     
=======================================
+ Hits        20094    20104   +10     
  Misses        383      383           
  Partials      577      577           

Files with missing lines	Coverage Δ
src/db/repositories.ts	96.12% <100.00%> (+0.01%)	⬆️
src/queue/processors.ts	88.42% <100.00%> (+0.03%)	⬆️

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