feat: ETag on /api/webhooks

[emmixeryng]

Overview

This PR adds strong ETag validators and 304 Not Modified support on GET /api/webhooks (Prometheus metrics scrape), reusing and extending src/middleware/etag.ts with a raw-body helper so the Prometheus Content-Type is preserved.

Related Issue

Closes #1110

Changes

⚡ ETag / 304 on webhooks metrics scrape

• [MODIFY] app/api/webhooks/route.ts

  • GET returns strong ETag + Cache-Control via withStrongEtagBody
  • Honours If-None-Match → 304
  • Restores missing applyRateLimit import (broken on main after an earlier merge)

• [MODIFY] src/middleware/etag.ts

  • Adds createStrongEtagFromBody / withStrongEtagBody for non-JSON representations

• [ADD] app/api/webhooks/webhooks.etag.test.ts

• [MODIFY] app/api/webhooks/route.test.ts / src/middleware/etag.test.ts

• [ADD] docs/api/webhooks-etag.md

Verification Results

npm run test -- --testPathPattern='webhooks.etag|middleware/etag\.test|webhooks/route\.test|exports.etag' --no-coverage
✅ webhooks.etag / etag middleware / webhooks route / exports.etag — all passed

Acceptance Criteria	Status
Strong ETag on /api/webhooks	✅ SHA-256 strong validator over metrics body bytes
304 when If-None-Match matches	✅ covered by tests (exact tag, list, and *)
Docs updated	✅ docs/api/webhooks-etag.md
Tests added and passing	✅ focused + regression green

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