x_exchange: new X token endpoint contract (client_type + client_secret in body)

OAuth phase 2: X/IG token exchange, claim_result in DB, callback redirects, hide claim buttons when claimed

Python 3.8 compatibility part 2: builtin generics -> typing generics ...

dict[str, X] / list[X] / tuple[X, Y] replaced with Dict/List/Tuple
across backend (main, oauth, scoring). Scoring tests pass.

Python 3.8 compatibility: X | None -> Optional[X] ...

Internal proxy VPS (deploy target) runs Ubuntu 20.04 / Python 3.8.
Replaced PEP 604 unions with typing.Optional across backend
(db, main, oauth, scoring, signing). Scoring tests still pass.

OG cards, deploy scripts, OAuth scaffolding ...

- backend/og.py: 1200x630 PNG result cards (Pillow, brutalist dark theme, cached per result_id)
- main.py: server-rendered /result/<id> and /v/<id> with dynamic OG meta injection (fixes missing /result route), /og/<id>.png card endpoint
- backend/oauth.py: X OAuth2 PKCE + Instagram FB-Login scaffolding, 503 until apps registered; /auth/x/* and /auth/ig/* routes wired
- scripts/deploy.sh: systemd unit + nginx + certbot, idempotent
- .env.example: all runtime + OAuth config vars
- live-tested: OG meta injection, PNG generation, 404s, OAuth 503 stubs