Document application, project and submission domain model

[TThanos3000]

Adds an RFC document describing the target domain model for applications, projects, submissions, teams, and evaluations.

Scope:

• documents why Application, Project, and Submission should be separate entities;
• proposes target fields, statuses, relations, constraints, and future endpoints;
• describes migration strategy that keeps existing Program/Project flows compatible;
• defines MVP implementation order for future backend and frontend work.

No models, views, serializers, workflows, deploy, nginx, or docker-compose files changed.