Relay's GET /admin page is served from src/admin-routes.ts - no behaviour change #121

First half of #94, made smaller (it failed twice on the deploy, not the change).

Done:

• [ ] A new Software/so101/src/admin-routes.ts exports one function that serves GET /admin: the page, and the 401/403 refusals. relay.ts calls it; the POST actions stay in relay.ts for now.
• [ ] Shared helpers (json, readBody) stay in relay.ts and are passed in as plain arguments.
• [ ] src/admin-relay.test.ts, src/relay.test.ts and src/admin.test.ts pass untouched; bun x tsc --noEmit passes.

The earlier attempt is on branch smith/94-relay-s-admin-routes-live-in-their-own-m (pull #107) and may be reused for the GET part.