feat(review): instrument impact-map + add a public AI-reuse-rate trend (#4448)

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Summary

• Completes feat(stats): AI-feature cache/reuse-efficiency rate, tracked over time, public #4448's cache-hit/miss instrumentation coverage and adds the aggregate public "how often we avoid redoing AI work" trend, both part of epic Epic: public, tracked-over-time benchmarks for gittensory's own review engine (speed, accuracy trend, efficiency) #4445.
• Instrumentation: adds cache hit/miss telemetry to impact-map's query cache (impact_map_query_cache, fix(review): impact-map re-embeds + re-queries the vector index up to 20x per pass with no result cache #4500) — the last of the genuinely cache-backed AI-touching capabilities that had zero reuse-rate signal, mirroring the established incr()+recordAuditEvent pattern exactly (repo_culture_profile fix(review): repo-culture-profile cache works correctly but has zero hit/miss telemetry #4509, grounding/review-memory shipped earlier this same epic in PR #4658). computeImpactMap now takes env as an explicit first argument (the existing RagInfra/StorageAdapter seam intentionally stays storage-only for portability, so telemetry is threaded separately).
• Two of the six capabilities the parent issue named — enrichment and reputation — are deliberately NOT instrumented: neither has a cache or any other "reuse a prior result" mechanism to measure. Enrichment always POSTs fresh to the external REES on every invocation (no local cache); reputation is a pure D1 aggregate with no caching layer (its own redundant-re-derivation issue was fixed differently, by threading a pre-computed value caller-to-callee in fix(review): reputation signal re-queried twice per pass with zero memoization #4507). Forcing a fabricated hit/miss signal onto either would misrepresent their real architecture and just show 100%-miss forever, adding noise instead of signal.
• Aggregate: new src/services/public-reuse-rate-trend.ts, extending GET /v1/public/stats with reuseRateTrend — 8 trailing weeks of hits/(hits+misses) across every github_app.*_cache_hit / *_cache_miss event (a naming convention, not a hardcoded capability list, so it automatically covers today's eight capabilities and any future one that follows the same convention) plus ai_review's three non-suffix-conforming reuse variants (frozen/paused/one-shot), which are the same "avoided a redundant AI call" signal under a different name.
• Deliberately not a cron/rollup table, mirroring feat(stats): review-accuracy as a historical trend, not just a live snapshot #4447's own design (PR #4655): audit_events is already durable, so a live day-bucketed re-query recomputes any historical week correctly on every request. Deliberately not scoped to the public-stats repo allowlist either (unlike accuracy) — a cache-hit/miss event carries no PR content or repo-specific outcome, so the aggregate number doesn't reveal per-repo activity, and target_key isn't uniformly shaped across all eight capabilities.
• Only the homepage//stats trend chart is deferred to a follow-up PR, matching the capture-then-extend split already used for feat(stats): review turnaround-time tracking (webhook received → comment posted), public + historical #4446 and feat(stats): review-accuracy as a historical trend, not just a live snapshot #4447.

Scope

• The PR title follows type(scope): short summary Conventional Commit format.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• Advances feat(stats): AI-feature cache/reuse-efficiency rate, tracked over time, public #4448 (part of epic Epic: public, tracked-over-time benchmarks for gittensory's own review engine (speed, accuracy trend, efficiency) #4445) — the frontend chart follows separately, so this does not close the issue.

Validation

• git diff --check
• npm run actionlint
• npm run typecheck
• npm run test:coverage locally — 100% line/branch coverage on both new/changed src/** files (impact-map.ts, public-reuse-rate-trend.ts), confirmed via scoped coverage runs
• npm run test:workers
• npm run build:mcp
• npm run test:mcp-pack
• npm run ui:openapi:check
• npm run ui:lint
• npm run ui:typecheck
• npm run ui:build
• npm audit --audit-level=moderate — 0 vulnerabilities
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries — hit/miss invariants, both fail-safe audit-write-failure directions, a falsy-project regression, and an end-to-end SQL-discrimination test confirming the aggregate query counts real event types (and only real event types) correctly

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. — N/A, no auth/session surface touched.
• API/OpenAPI/MCP behavior is updated and tested where needed — PublicStatsSchema extended with reuseRateTrend, regenerated via npm run ui:openapi, settings-parity checked.
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks. — N/A, backend-only change (no frontend chart in this slice).
• Visible UI changes include a UI Evidence section. — N/A, backend-only change, no visible UI.
• Public docs/changelogs are updated where needed. — N/A, no changelog edits in a normal PR; no public docs yet since there's no chart consuming it until the follow-up PR.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	gittensory-ui	f580243	Commit Preview URL Branch Preview URL	Jul 10 2026, 11:17 AM

[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 94.14%. Comparing base (7e76b2e) to head (f580243).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4671   +/-   ##
=======================================
  Coverage   94.13%   94.14%           
=======================================
  Files         434      435    +1     
  Lines       38542    38576   +34     
  Branches    14056    14060    +4     
=======================================
+ Hits        36282    36316   +34     
  Misses       1601     1601           
  Partials      659      659           

Files with missing lines	Coverage Δ
src/api/routes.ts	94.26% <100.00%> (ø)
src/openapi/schemas.ts	100.00% <ø> (ø)
src/queue/processors.ts	95.29% <100.00%> (ø)
src/review/impact-map.ts	100.00% <100.00%> (ø)
src/services/public-reuse-rate-trend.ts	100.00% <100.00%> (ø)

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