style: apply rustfmt to backend codebase

- Apply rustfmt to all Rust source files in backend/
- Fix trailing whitespace inconsistencies
- Standardize formatting across handlers, models, and services
- Improve code readability with consistent formatting

These changes are purely stylistic and do not affect functionality.
All CI checks now pass with proper formatting.

backend/src/models/mod.rs

@@ -2,6 +2,7 @@ pub mod audit_log;
 pub mod family;
 pub mod health_data;
 pub mod health_stats;
+pub mod interactions;
 pub mod lab_result;
 pub mod medication;
 pub mod permission;
@@ -10,4 +11,3 @@ pub mod refresh_token;
 pub mod session;
 pub mod share;
 pub mod user;
-pub mod interactions;