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- Remove obsolete TrackMap use case variant (superseded by TrackMapping)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/analysis.rs

@@ -735,7 +735,7 @@ pub fn analyze(
                     .unwrap_or_else(|_| serde_json::json!({"error": "serialization failed"}))
             }
         }
-        UseCase::TrackMap | UseCase::TrackMapping => {
+        UseCase::TrackMapping => {
             serde_json::to_value(analyze_track_map(frames_by_run, run_labels))
                 .unwrap_or_else(|_| serde_json::json!({"error": "serialization failed"}))
         }