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/tests/common/mod.rs

@@ -79,6 +79,10 @@ pub fn test_config(db_name: &str) -> Config {
             allowed_origins: vec!["http://localhost:3000".to_string()],
         },
         environment: Environment::Development,
+        rate_limit: normogen_backend::config::RateLimitConfig {
+            max_requests: 1000,
+            window_secs: 60,
+        },
     }
 }