Relay's POST /admin actions (save, mod-add, mod-remove, pause, resume, end, take) are handled in src/admin-routes.ts - no behaviour change #122

Second half of #94.

Done:

• [ ] POST /admin handling, the 16 KiB form limit and every action (save, mod-add, mod-remove, pause, resume, end, take) move from Software/so101/src/relay.ts into src/admin-routes.ts; relay.ts has no admin logic left beyond the one call.
• [ ] src/admin-relay.test.ts, src/relay.test.ts and src/admin.test.ts pass untouched; bun x tsc --noEmit passes.