GET /api/v1/repos/:owner/:name/reports carries an ETag and answers a repeat with 304 until a report arrives or is answered #9203

The inbox polls waiting reports; while nothing new waits it costs a header.

Done looks like:

• [x] The /api/v1/repos/:owner/:name/reports route in src/http/api/reports.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 report is filed, answered or leaves the window asked for, and a different window gets its own etag; two viewers who are shown different bodies never share an etag.
• [x] A token short of rw, a bad window 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/reports-etag.test.ts covers each of the above and fails without the change.

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

Reviewed pull #9230 at its head: ticked The /api/v1/repos/:owner/:name/reports route in src/http/api/reports.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 report is filed, answered or leaves the window asked for, and a different window gets its own etag; two viewers who are shown different bodies never share an etag.; A token short of rw, a bad window 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/reports-etag.test.ts covers each of the above and fails without the change..