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/src/main.rs

@@ -12,7 +12,7 @@ use doctate_server::analyze;
 use doctate_server::config::Config;
 use doctate_server::gazetteer::{Gazetteer, SpellbookDict};
 use doctate_server::transcribe;
-use doctate_server::{AppState, OnelinerWatermark};
+use doctate_server::AppState;
 
 #[tokio::main]
 async fn main() {
@@ -110,13 +110,6 @@ async fn main() {
         }
     });
 
-    // Oneliner watermark: per-user "last successful oneliner write (UTC)".
-    // Feeds the ETag on `/api/oneliners`. Shared across the transcribe
-    // worker (writer) and the oneliners route handler (reader).
-    let oneliner_watermark: OnelinerWatermark = Arc::new(tokio::sync::RwLock::new(
-        std::collections::HashMap::new(),
-    ));
-
     // Transcription pipeline: channel + worker + recovery scan.
     let (transcribe_tx, transcribe_rx) = transcribe::channel();
     let transcribe_busy: doctate_server::WorkerBusy =
@@ -127,7 +120,6 @@ async fn main() {
         http_client.clone(),
         transcribe_busy.clone(),
         vocab.clone(),
-        oneliner_watermark.clone(),
     ));
     {
         let tx = transcribe_tx.clone();
@@ -135,13 +127,12 @@ async fn main() {
         let client = http_client.clone();
         let cfg = config.clone();
         let vocab = vocab.clone();
-        let watermark = oneliner_watermark.clone();
         tokio::spawn(async move {
             transcribe::recovery::scan_and_enqueue(&data_path, &tx).await;
             // Then catch up oneliners for cases that crashed between
             // transcript-write and oneliner-write in a previous run.
             transcribe::recovery::regenerate_missing_oneliners(
-                &data_path, &client, &cfg, &vocab, &watermark,
+                &data_path, &client, &cfg, &vocab,
             )
             .await;
         });
@@ -173,7 +164,6 @@ async fn main() {
         session_store: doctate_server::web_session::new_store(),
         analyze_busy: doctate_server::AnalyzeBusy(analyze_busy),
         transcribe_busy: doctate_server::TranscribeBusy(transcribe_busy),
-        oneliner_watermark,
     };
 
     let addr = format!("0.0.0.0:{}", config.server_port);