GET /api/v1/repos/:owner/:name/following/proposals carries an ETag and answers a repeat with 304 until a proposal to the parent changes #9246

A fork watching what it proposed to its parent gets a 304 while no proposal moved.

Done looks like:

• [x] The /following/proposals GET route in src/http/api/following.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• [x] The etag moves when a proposal is made, answered, landed or withdrawn; two viewers shown different bodies never share an etag.
• [x] A missing repository, not_a_fork, and access refusals answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.
• [x] test/following-proposals-etag.test.ts covers each of the above, each case failing without the change.

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

Reviewed pull #9297 at its head: ticked The /following/proposals GET route in src/http/api/following.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headers, no body and the securityFloor headers (holds).; The etag moves when a proposal is made, answered, landed or withdrawn; two viewers shown different bodies never share an etag.; A missing repository, not_a_fork, and access refusals answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.; test/following-proposals-etag.test.ts covers each of the above, each case failing without the change..