Add a pre-computed scenario pack to the TRIBE v2 server

[slavazeph-coder]

Layer 3 — brainsnn-r3f-app/server/scenarios/

Drop in a new scenario (e.g. musical_improv, dreaming, argument) as a .pkl or JSON of per-region activity frames over time. Wire it into the /scenarios endpoint in server/api.py so the SPA can select it.

See server/precompute.py for how the existing scenarios were generated, and server/README.md for running the FastAPI backend locally.