refactor(api)!: remove the shared operator secret, and let hosts sync their own

A single secret that turned any request into an operator was the only
credential several routes accepted, and it had no caller left: the device
pairing it existed for is on hold, and nothing in this tree or any client
sent it. What remained was a key that bypassed authentication entirely,
required to boot, and checked by nobody.

Every route behind it had a better answer available:

- Library and game sync move to host credentials. Both took a `userId` in
  the body, which meant one secret could write into anybody's library. A
  host now says which of its enrolled users a batch is for, and that claim
  is checked against the Steam sign-ins it actually holds — one box carries
  several people's accounts, so the pair is the unit.
- Download-state reporting narrows to hosts alone, and the body that could
  name a different host is gone. Which host is reporting comes from its own
  credentials, and a body that still names one is refused rather than
  ignored.
- Linking a Steam account is always for the caller.
- Creating a game by hand is deleted; syncing already upserts the catalogue.
- Reading the waitlist is deleted. Every address on it belongs to someone
  who has not agreed to anything, and answering it over HTTP made that list
  something a leaked key could drain.
- The pairing-code routes are deleted with the flow they served. The domain
  module and its table stay, so returning to it is a route file rather than
  a migration.

Nothing in the API now accepts a credential that stands for more than one
caller: every request resolves to a specific user or a specific host, which
is what lets a route say "the caller's own library" and mean it.

BREAKING CHANGE: the `x-nestri-admin-token` header is no longer accepted and
`ADMIN_SHARED_SECRET` is no longer read. `POST /games`, `GET /waitlist` and
the `/pairing-code` routes are gone; `POST /games/sync` and `POST /library/sync`
now require host credentials and take `userId` in the body; `POST /steam/link`
no longer accepts `userId`; `POST /games/download-state` no longer accepts
`hostId`.

apps/api/app/index.ts

@@ -15,7 +15,6 @@ import { GameApi } from './routes/game.js';
 import { IndexApi } from './routes/index.js';
 import { LibraryApi } from './routes/library.js';
 import { MachineApi } from './routes/machine.js';
-import { PairingCodeApi } from './routes/pairing-code.js';
 import { SessionApi } from './routes/session.js';
 import { SteamApi } from './routes/steam.js';
 import { UserApi } from './routes/user.js';
@@ -43,7 +42,6 @@ const routes = app
 	.route('/steam', SteamApi.route)
 	.route('/library', LibraryApi.route)
 	.route('/games', GameApi.route)
-	.route('/pairing-code', PairingCodeApi.route)
 	.route('/machine', MachineApi.route)
 	.route('/machine', SessionApi.machineRoute)
 	.route('/machine', EnrolmentApi.route)
@@ -125,7 +123,6 @@ export type ApiEnv = {
 	AUTH_INTERNAL_URL?: string;
 	HYPERDRIVE?: Hyperdrive;
 	DATABASE_URL?: string;
-	ADMIN_SHARED_SECRET?: string;
 };
 
 export default {