fix(ci): allowlist GET /content/channel-detail/:slug (route-drift guard)

[jung-thomas]

Problem

Unit-tests CI is red on DEV (and on the merged #2169 branch). The failing step is the check-srv-qa-route-drift static build guard, not a test assertion:

[check-srv-qa-route-drift] FAILED — /content/* route drift detected:
  Routes on srv but MISSING from srv-qa/server.js:
    GET /content/channel-detail/:slug    (srv/server.js:829)

Cause

#2169 (Channel Atlas / channels-hub Phase 2–3) added the GET /content/channel-detail/:slug serve route to srv/server.js but not to srv-qa/server.js. The guard enforces /content/* route parity between the two channels unless a route is allowlisted.

Fix

channel-detail is a public prod content surface (channel-prefixed BLOBs, lowercase-canonicalize + 301), analogous to GET /content/topics/:slug — not a tutorial-draft author-preview endpoint. Its publish sibling POST /content/publish/render-channels is already in ALLOWLIST_ONLY_ON_SRV; #2169 simply missed the serve counterpart. This adds it with the matching justification.

Verification

npx tsx scripts/check-srv-qa-route-drift.ts
[check-srv-qa-route-drift] OK — 35 /content/* route(s) on srv, 13 on srv-qa (22 srv-only + 0 srv-qa-only allowlisted).