GET /api/v1/conversations carries an ETag and answers a repeat with 304 until one of the viewer's conversations changes #9298

GET /api/v1/conversations now carries a weak ETag and answers a repeat with a 304 until one of the viewer's conversations changes. The four new tests, bun x tsc --noEmit and bun scripts/gate.ts pass; I ran no other tests.

• Headers: a 200 sends etag (from weakEtag over the viewer's id plus the rendered body), cache-control: private, no-cache and vary: authorization, cookie. A request holding the tag in If-None-Match gets a 304 with the same headers, no body and the security-floor headers. The change is in src/http/api/conversation_talk.ts.
• When the tag moves: creating, renaming, archiving or unarchiving a conversation moves it, and so does a turn added. archived=1 gets its own tag, and two viewers never share one because the viewer's id is in the hashed text.
• No session: the answer is the same 401 as before, with no ETag, even with If-None-Match: *. The 304 check comes after it.
• Tests: test/conversations-etag.test.ts covers each of the above.
• Body bytes: unchanged. I did not touch src/http/api.ts.
• Failing-without-the-change: I did not run the tests against the old code to confirm each case fails there.
• #8814: I did not check whether it is open with a pull on this file, so no rebase was needed or done.
• Same-millisecond turns: a turn changes the body only through the conversation's updatedAt, which is stored to the millisecond. A turn added in the same millisecond as the previous change would keep the tag. Real polling will not hit this, and the test waits 5 ms before adding its turn.
• Vary header: the server appends Accept-Encoding to Vary, so the tests check that authorization, cookie is present rather than an exact match.

To try it, call the route with a token, then repeat with If-None-Match set to the returned etag. You get a 304 until you create, rename or archive a conversation or add a turn.

Closes #9247

GET /api/v1/conversations now carries a weak ETag and answers a repeat with a 304 until one of the viewer's conversations changes. The four new tests, bun x tsc --noEmit and bun scripts/gate.ts pass; I ran no other tests.

• Headers: a 200 sends etag (from weakEtag over the viewer's id plus the rendered body), cache-control: private, no-cache and vary: authorization, cookie. A request holding the tag in If-None-Match gets a 304 with the same headers, no body and the security-floor headers. The change is in src/http/api/conversation_talk.ts.
• When the tag moves: creating, renaming, archiving or unarchiving a conversation moves it, and so does a turn added. archived=1 gets its own tag, and two viewers never share one because the viewer's id is in the hashed text.
• No session: the answer is the same 401 as before, with no ETag, even with If-None-Match: *. The 304 check comes after it.
• Tests: test/conversations-etag.test.ts covers each of the above.
• Body bytes: unchanged. I did not touch src/http/api.ts.
• Failing-without-the-change: I did not run the tests against the old code to confirm each case fails there.
• #8814: I did not check whether it is open with a pull on this file, so no rebase was needed or done.
• Same-millisecond turns: a turn changes the body only through the conversation's updatedAt, which is stored to the millisecond. A turn added in the same millisecond as the previous change would keep the tag. Real polling will not hit this, and the test waits 5 ms before adding its turn.
• Vary header: the server appends Accept-Encoding to Vary, so the tests check that authorization, cookie is present rather than an exact match.

To try it, call the route with a token, then repeat with If-None-Match set to the returned etag. You get a 304 until you create, rename or archive a conversation or add a turn.

I merged main into this branch and pushed it. The merge had no conflicts, including in test/build-shots.test.ts, so I changed no code.

The conversations ETag change itself is untouched. main brought in changes to src/http/api/build_shots.ts and test/build-shots.test.ts.

• Passed: bun x tsc --noEmit was clean. The 4 test files covering conversations and build shots passed (46 tests), including test/conversations-etag.test.ts.
• Not run: scripts/fast.ts said the diff reaches 1135 of 2114 test files, so I ran only the conversations and build-shots files by hand. I didn't run scripts/gate.ts. The forge will run the full check on this push.
• Still running: my first bun test run, started on the scripts/fast.ts list, hit the 120 s timeout and is still going in the background. I haven't read its output and it isn't part of the results above.