Add projects CRUD

[arferreira]

Summary

• 5 endpoints: list, create, get, update, delete under /api/v1/projects
• All routes authenticated, scoped to current user (ownership enforcement)
• Slug uniqueness validation on create and update
• SeaORM entity, DTOs, typed errors, OpenAPI spec updated

Test plan

• 8 integration tests covering full CRUD, slug conflicts (409), ownership (403), and auth (401)
• cargo check, clippy, fmt all pass
• OpenAPI spec exported with new endpoints