GET /api/v1/keeper carries an ETag and answers a repeat with 304 until a release is kept, promoted or rolled back #9204

The release history is polled by the owner's board; while no release moved it is a 304.

Done looks like:

• [x] The /api/v1/keeper route in src/http/api/keeper.ts answers 200 with a weak etag from weakEtag (src/http/etag.ts) over the rendered body, cache-control: private, no-cache and vary: authorization, cookie.
• [x] A repeat holding it in If-None-Match is a 304 with the same headers, no body and the securityFloor headers (holds).
• [x] The etag moves when a release is recorded, promoted, held or rolled back; two viewers who are shown different bodies never share an etag.
• [x] A viewer who is neither an administrator nor a repository reader (the 403) answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.
• [x] test/keeper-etag.test.ts covers each of the above and fails without the change.

Files: src/http/api/keeper.ts, test/keeper-etag.test.ts. Risks: do not touch src/http/api.ts; the body stays byte-for-byte as it is.

Reviewed pull #9231 at its head: ticked The /api/v1/keeper route in src/http/api/keeper.ts answers 200 with a weak etag from weakEtag (src/http/etag.ts) over the rendered body, cache-control: private, no-cache and vary: authorization, cookie.; A repeat holding it in If-None-Match is a 304 with the same headers, no body and the securityFloor headers (holds).; The etag moves when a release is recorded, promoted, held or rolled back; two viewers who are shown different bodies never share an etag.; A viewer who is neither an administrator nor a repository reader (the 403) answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.; test/keeper-etag.test.ts covers each of the above and fails without the change..