[codex] Add ETags for events and stats

[pq198363-ops]

Summary

Closes #145.

• Add a shared etagFor helper for weak cache validators.
• Reuse the helper for the existing services list ETag path.
• Add explicit weak ETags and If-None-Match 304 handling for GET /api/v1/events and GET /api/v1/stats.
• Scope events validators to the effective since, type, and limit query values so different filtered poll requests cannot collide when their bodies are currently identical.
• Document conditional polling support in the README.

RED/GREEN

RED before implementation:

npm run build
NODE_ENV=test node --test dist/etag-events-stats.test.js

fail 1
uses query-specific events ETags even when filtered bodies match
actual and expected both: W/"c-F6g7sByJuuOcYYkgTnzj94yg+ZE"

GREEN after implementation:

npm run build
NODE_ENV=test node --test dist/etag-events-stats.test.js

tests 4
pass 4
fail 0

Verification

npx prettier --check README.md src/httpCache.ts src/routes/events.ts src/routes/metrics.ts src/routes/services.ts src/etag-events-stats.test.ts
All matched files use Prettier code style!

npm run lint
eslint .

npm run build
NODE_ENV=test node --test dist/etag-events-stats.test.js
tests 4
pass 4
fail 0

NODE_ENV=test node --test dist/*.test.js dist/**/*.test.js
tests 93
pass 93
fail 0

git diff --check

Note: on Windows PowerShell I used the direct $env:NODE_ENV='test'; node --test ... equivalent after npm run build because the package npm test script uses POSIX-style NODE_ENV=test syntax.