feat(scenarios,environments): add markdown description field

- add optional description to scenario and environment entities
- expose description in create/update DTOs, MCP tools, and export DTOs
- render description as markdown on detail pages
- add description editor (CodeEditor) to create/edit forms for both resources

server/src/scenario/scenario.entity.ts

@@ -17,6 +17,9 @@ export class ScenarioEntity {
   @Column()
   name: string;
 
+  @Column("text", { nullable: true })
+  description: string | null;
+
   @OneToMany(() => ScenarioStepEntity, (step) => step.scenario, {
     cascade: true,
     eager: false,