Add utoipa and Scalar for API documentation

- Add utoipa and utoipa-scalar dependencies
- Add ToSchema derives to all models
- Add OpenAPI path annotations to auth, users, and health handlers
- Create openapi.rs module with API documentation
- Serve Scalar UI at /docs endpoint
- Include JWT bearer auth security scheme

Co-Authored-By: Warp <agent@warp.dev>

Cargo.toml

@@ -50,6 +50,10 @@ rand = "0.8"
 base64 = "0.22"
 libm = "0.2"  # Math functions for rating calculations
 
+# API Documentation
+utoipa = { version = "4", features = ["axum_extras", "chrono"] }
+utoipa-scalar = { version = "0.1", features = ["axum"] }
+
 # S3 compatible storage
 aws-sdk-s3 = "1"
 aws-config = "1"