feat: Add doctate-client-core crate

Introduces a new crate `doctate-client-core` to house shared client-side
logic. This includes:

- `case_store`: Manages local case marker files and merging with server
  snapshots.
- `snapshot_cache`: Placeholder for caching server responses.
- `oneliner_poller`: Placeholder for the background polling task.

The workspace configuration and `Cargo.lock` have been updated to
include the new crate.

server/src/routes/oneliners.rs

@@ -18,7 +18,8 @@ use axum::extract::{Query, State};
 use axum::http::{header, HeaderMap, StatusCode};
 use axum::response::{IntoResponse, Response};
 use axum::Json;
-use serde::{Deserialize, Serialize};
+use doctate_common::oneliners::{OnelinerEntry, OnelinersResponse};
+use serde::Deserialize;
 use time::format_description::well_known::Rfc3339;
 use time::{Duration, OffsetDateTime};
 use tracing::warn;
@@ -35,21 +36,6 @@ pub struct OnelinersQuery {
     hours: Option<u32>,
 }
 
-#[derive(Serialize)]
-struct OnelinersResponse {
-    as_of: String,
-    window_hours: u32,
-    oneliners: Vec<OnelinerEntry>,
-}
-
-#[derive(Serialize)]
-struct OnelinerEntry {
-    case_id: String,
-    oneliner: Option<String>,
-    created_at: String,
-    updated_at: Option<String>,
-}
-
 pub async fn handle_oneliners(
     user: AuthenticatedUser,
     State(watermark): State<OnelinerWatermark>,