feat(backend): dose scheduling + health stats zero-knowledge

Backend changes (frontend + tests follow in next commit):

Dose scheduling:
- New DoseSchedule struct (times_per_day + days_of_week) as a top-level
  plaintext field on Medication. Create/update requests accept it.
- Revised get_adherence: computes scheduled_doses from the schedule over the
  period (days_of_week filtering), so missed doses are now reflected.
  Falls back to taken/total_logged when no schedule.

Health stats zero-knowledge:
- HealthStatistic model now uses opaque encrypted_data blob (like medications).
  Only recorded_at stays plaintext (filterable/sortable).
- HealthStatResponse wire type. Handlers echo opaque blobs.
- Removed the trends endpoint (server can't compute trends on ciphertext;
  frontend computes them client-side after decrypting).
- Deleted the dead HealthData model (kept EncryptedField which it defined).

Verified: backend 24 tests, 0 clippy warnings.

backend/src/app.rs

@@ -72,7 +72,6 @@ pub fn build_app(state: AppState) -> Router {
         // Health statistics management (Phase 2.7)
         .route("/api/health-stats", post(handlers::create_health_stat))
         .route("/api/health-stats", get(handlers::list_health_stats))
-        .route("/api/health-stats/trends", get(handlers::get_health_trends))
         .route("/api/health-stats/:id", get(handlers::get_health_stat))
         .route("/api/health-stats/:id", put(handlers::update_health_stat))
         .route("/api/health-stats/:id", delete(handlers::delete_health_stat))