GET /api/v1/admin/drills carries an ETag and answers a repeat with 304 until a drill runs or its result changes #9205

The resilience page's failure domains are re-read on every visit; a repeat with nothing drilled is a 304.

Done looks like:

• [x] The /api/v1/admin/drills route in src/http/api/drills.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 drill starts, finishes, or a failure domain's state changes; two viewers who are shown different bodies never share an etag.
• [x] No session and a viewer who is not a person admin (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/drills-etag.test.ts covers each of the above and fails without the change.

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

Reviewed pull #9233 at its head: ticked The /api/v1/admin/drills route in src/http/api/drills.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 drill starts, finishes, or a failure domain's state changes; two viewers who are shown different bodies never share an etag.; No session and a viewer who is not a person admin (the 403) answer exactly as today, with no ETag, even with If-None-Match: *; the 304 is reached only after the access check.; test/drills-etag.test.ts covers each of the above and fails without the change..