GET /api/v1/repos/:owner/:name/upstream carries an ETag and answers a repeat with 304 until the outside upstream or its last check changes #9245

A repository following an outside upstream is checked by its admin and agents; while the upstream and its last check are unchanged, a repeat is a 304.

Done looks like:

• [x] The /upstream GET route in src/http/api/upstream.ts answer 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 the upstream is set, changed or removed, or a check records a new result; two viewers shown different bodies never share an etag.
• [x] A token short of ro, a non-admin and a missing repository answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.
• [x] test/upstream-etag.test.ts covers each of the above, each case failing without the change.

Files: src/http/api/upstream.ts, test/upstream-etag.test.ts. Risks: do not touch src/http/api.ts; bodies stay byte-for-byte as they are.

Reviewed pull #9267 at its head: ticked The /upstream GET route in src/http/api/upstream.ts answer 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 the upstream is set, changed or removed, or a check records a new result; two viewers shown different bodies never share an etag.; A token short of ro, a non-admin and a missing repository answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.; test/upstream-etag.test.ts covers each of the above, each case failing without the change..