Refactor ETag generation for oneliners API

Remove the per-user oneliner watermark, which was previously used to
generate ETags for the `/api/oneliners` endpoint. The watermark was
intended to track the last successful oneliner write for each user.

The ETag generation has been refactored to use a deterministic FNV-1a
hash. This hash is computed over a sorted list of tuples containing
`(case_id, last_recording_at_ns, oneliner_mtime_ns)` for all visible
cases. This approach ensures that the ETag changes whenever any
listen-relevant file system modification occurs, such as a new
recording, oneliner regeneration, soft-delete, undo, or
upload-auto-reopen. This new method is more robust and avoids the
complexity of managing per-user state.

The `OnelinerWatermark` type alias and its associated logic have been
removed from `AppState` and the relevant modules (`transcribe::worker`,
`transcribe::recovery`, `routes::oneliners`, `main`). New integration
tests have been added to verify that various delete operations (single
delete, undo delete, bulk delete) correctly trigger an ETag update,
ensuring cache invalidation.

server/tests/oneliners_api_test.rs

@@ -128,7 +128,8 @@ async fn empty_user_dir_returns_200_empty_list() {
     assert_eq!(response.status(), StatusCode::OK);
 
     let etag = header_str(&response, "etag");
-    assert!(etag.starts_with("W/\"0-16\""), "etag was {etag:?}");
+    assert!(etag.starts_with("W/\""), "etag was {etag:?}");
+    assert!(etag.ends_with("-16\""), "etag was {etag:?}");
 
     let body = body_json(response).await;
     assert_eq!(body["window_hours"], 16);