feat: Add API endpoint for listing oneliners

This commit introduces a new API endpoint `/api/oneliners` that allows
authenticated users to retrieve a list of their recent oneliners.

The endpoint supports conditional GET requests using the `ETag` header
for efficient caching. It also allows specifying a time window for the
oneliners via the `hours` query parameter, with sensible defaults and
clamping.

The implementation includes:
- A new route handler `handle_oneliners`.
- A new type `OnelinerWatermark` for tracking user-specific timestamps.
- Logic for scanning user data directories, filtering by time, and
  handling deleted cases.
- Test cases to cover various scenarios, including caching, time
  windowing, and edge cases.

server/src/routes/mod.rs

@@ -3,6 +3,7 @@ mod case_actions;
 mod debug;
 mod health;
 mod login;
+mod oneliners;
 mod upload;
 pub(crate) mod user_web;
 pub(crate) mod web;
@@ -17,6 +18,7 @@ pub fn api_router() -> Router<AppState> {
         .route("/api/health", get(health::handle_health))
         .route("/api/debug/whoami", get(debug::handle_whoami))
         .route("/api/upload", post(upload::handle_upload))
+        .route("/api/oneliners", get(oneliners::handle_oneliners))
         .route("/web/login", get(login::handle_login_page).post(login::handle_login_submit))
         .route("/web/logout", post(login::handle_logout))
         .route("/web/cases", get(user_web::handle_my_cases))