feat: implement health statistics tracking (Phase 2.7 Task 2)

- Add HealthStatistics model with 10 stat types
- Implement HealthStatisticsRepository
- Create 6 health stats API endpoints
- Add trend analysis with summary calculations
- Follow medication repository pattern

Status: 60% complete, needs compilation fixes

backend/src/main.rs

@@ -78,6 +78,7 @@ async fn main() -> anyhow::Result<()> {
 
     // Get the underlying MongoDB database for security services
     let database = db.get_database();
+    let mongo_client = database.client().clone();
 
     // Initialize security services (Phase 2.6)
     let audit_logger = security::AuditLogger::new(&database);
@@ -92,6 +93,12 @@ async fn main() -> anyhow::Result<()> {
         1440, // max_duration_minutes (24 hours)
     );
 
+    // Initialize health stats repository (Phase 2.7) - using Collection pattern
+    let health_stats_collection = database.collection("health_statistics");
+    let health_stats_repo = models::health_stats::HealthStatisticsRepository::new(
+        health_stats_collection
+    );
+
     // Create application state
     let state = config::AppState {
         db,
@@ -100,8 +107,10 @@ async fn main() -> anyhow::Result<()> {
         audit_logger: Some(audit_logger),
         session_manager: Some(session_manager),
         account_lockout: Some(account_lockout),
+        health_stats_repo: Some(health_stats_repo),
+        mongo_client: Some(mongo_client),
     };
-
+    
     eprintln!("Building router with security middleware...");
     
     // Build public routes (no auth required)
@@ -146,6 +155,14 @@ async fn main() -> anyhow::Result<()> {
         .route("/api/medications/:id/delete", post(handlers::delete_medication))
         .route("/api/medications/:id/log", post(handlers::log_dose))
         .route("/api/medications/:id/adherence", get(handlers::get_adherence))
+        
+        // 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))
         .layer(axum::middleware::from_fn_with_state(
             state.clone(),
             middleware::jwt_auth_middleware