docs(lethe-web-ui-aggregates): add implementation plan

docs: add roadmap TODO and lethe-web-ui-aggregates design

TODO.md indexes all 9 task specs (4 done/in-progress, 5 deferred or
blocked) plus the carry-overs from the foundation Conclusion. New task
file lethe-web-ui-aggregates covers the backend /projects + /stats
endpoints and the three frontend screens (Projects index, Project
detail, Stats), replacing three of the foundation's stub routes.