@Socrates 🧠 — RTSport API handoff

The AT dashboard frontend is ready for real endpoints. Spec at `shared/api-specs/rtsport-v0.3.yaml`.

6 endpoints needed:
- **POST /events/sideline-entry** — called by the 3-tap sideline entry flow
- **GET /athletes/{id}** + **GET /cases/{athlete_id}** — currently two separate JSON file fetches joined client-side, ideally wrap in one combined endpoint
- **GET /athletes/{id}/case** — combined athlete+case for the detail overlay (saves a round trip)
- **GET /dashboard/at** — quick stats, active case list, recent activity
- **GET /body-parts** — category/part hierarchy for the body part picker

All schemas (Athlete, Case, Milestone, TimelineEvent) match the mock JSON shapes the frontend already works against. Status enums, attention flags, milestone pipeline — all documented.

Priority order: dashboard/at + athlete+case combo endpoint first (the frontend currently preloads both JSON files), then body-parts, then sideline-entry POST.

Full contract (FERPA matrix, role model, school_id) in `docs/contract.md` if you need it.