feat: persistent sessions, remove track_map use case, car characteristics DB

- Remove obsolete TrackMap use case variant (superseded by TrackMapping)
- Add session/run/analysis persistence: metadata to data/sessions/<id>.json,
  frames to <id>.frames.json (split for lazy loading)
- Atomic writes (temp + rename), compact JSON, off-thread frame serialization
- load_all loads metadata only at startup; ensure_frames_loaded lazy-loads
  frames before analysis
- Normalize Recording runs to Stopped on reload (stale ring-buffer markers)
- Cache car_name on UseCaseSession during run finalization
- Add characteristics.rs: aggregate shift-point data per car across sessions
- New API: GET /api/characteristics/cars, GET /api/characteristics/cars/{car}
- Add Deserialize to TelemetryFrame and FeedSource for session round-trip

src/api.rs

@@ -14,6 +14,7 @@ use std::sync::{Arc, Mutex, MutexGuard};
 use tower_http::cors::CorsLayer;
 
 use crate::analysis;
+use crate::characteristics;
 use crate::feed_state::FeedState;
 use crate::recording::{Game, RecordingManager, RecordingStatus, StartRecordingRequest, UseCase};
 use crate::session::SessionManager;
@@ -267,7 +268,7 @@ async fn run_analysis(
     Path(id): Path<String>,
 ) -> Response {
     let (use_case, frames_by_run, run_labels, recording_id) = {
-        let mgr = lock_sessions(&state);
+        let mut mgr = lock_sessions(&state);
         match mgr.get_session(&id) {
             Some(session) => {
                 if session.status != crate::session::SessionStatus::Completed {
@@ -681,6 +682,24 @@ async fn list_games() -> Response {
     ok_json(Game::valid_choices())
 }
 
+async fn list_car_profiles(
+    State(state): State<Arc<AppState>>,
+) -> Response {
+    let mgr = lock_sessions(&state);
+    ok_json(characteristics::car_profiles(&mgr))
+}
+
+async fn get_car_profile(
+    State(state): State<Arc<AppState>>,
+    Path(car): Path<String>,
+) -> Response {
+    let mgr = lock_sessions(&state);
+    match characteristics::car_profile(&mgr, &car) {
+        Some(profile) => ok_json(profile),
+        None => err(StatusCode::NOT_FOUND, &format!("No data for car '{}'", car)),
+    }
+}
+
 pub fn build_router(state: Arc<AppState>) -> Router {
     Router::new()
         .route("/api/sessions", get(list_sessions).post(create_session))
@@ -714,6 +733,8 @@ pub fn build_router(state: Arc<AppState>) -> Router {
         .route("/api/tracks/build", post(build_track))
         .route("/api/use-cases", get(list_use_cases))
         .route("/api/games", get(list_games))
+        .route("/api/characteristics/cars", get(list_car_profiles))
+        .route("/api/characteristics/cars/{car}", get(get_car_profile))
         .layer(CorsLayer::permissive())
         .with_state(state)
 }