feat(api): auto-generate HTTP handlers with OpenAPI documentation

[RAprogramm]

Summary

Implement automatic HTTP handler generation with full OpenAPI/Swagger documentation via utoipa integration.

Motivation

Currently users manually write HTTP handlers. With commands already generating business logic entry points, we can auto-generate the HTTP layer with professional API documentation.

Proposed Syntax

#[derive(Entity)]
#[entity(
    table = "users",
    commands,
    events,
    api(
        tag = "Users",
        tag_description = "User management API",
        path_prefix = "/api/v1",
        security = "bearer",
        public = [Register],
    )
)]
pub struct User {
    #[id]
    id: Uuid,
    
    /// User email address
    #[field(create, update, response)]
    #[validate(email, max_length = 255)]
    #[example = "user@example.com"]
    email: String,
    
    #[command(Register, summary = "Register new user")]
    #[command(Deactivate, summary = "Deactivate user")]
}

Generated Output

• Axum handlers with #[utoipa::path] annotations
• OpenAPI schemas for all DTOs with descriptions, constraints, examples
• Router factory function
• OpenApi struct for Swagger UI

Sub-tasks

• feat(api): parse api(...) entity attributes #76 Parse api(...) attributes
• feat(api): generate HTTP handlers from commands #77 Generate handlers from commands
• feat(api): doc comments to OpenAPI descriptions #78 Doc comments to OpenAPI descriptions
• feat(api): validation attributes to OpenAPI constraints #79 Validation to OpenAPI constraints
• feat(api): example values for OpenAPI schemas #80 Examples support via #[example]
• feat(api): security configuration for endpoints #81 Security configuration
• feat(api): auto-document error responses #82 Error type documentation
• feat(api): API versioning support #83 API versioning support

Dependencies

• utoipa
• utoipa-axum