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/middleware/auth.ts

@@ -85,11 +85,6 @@ function getClient(env: Record<string, unknown>) {
 }
 
 export const auth: MiddlewareHandler = async (c, next) => {
-	const adminToken = c.req.header('x-nestri-admin-token');
-	if (adminToken && adminToken === Env.get().ADMIN_SHARED_SECRET) {
-		return Actor.with({ type: 'admin', properties: {} }, next);
-	}
-
 	// A registered nessh host proves it is itself, rather than asserting an id
 	// nobody checks. Wrong credentials fall through to public rather than
 	// erroring, so probing tells an attacker nothing about which ids exist.
@@ -263,34 +258,3 @@ export const machineOnly: MiddlewareHandler = async (_, next) => {
 	}
 	return next();
 };
-
-/**
- * A box reporting about itself, or an operator reaching in.
- *
- * The two are not equivalent and routes behind this must not treat them so: a
- * machine may only speak for itself, while admin still has to say which host
- * it means. Keeping admin is what lets an operator repair state by hand.
- */
-export const machineOrAdmin: MiddlewareHandler = async (_, next) => {
-	const actor = Actor.use();
-	if (actor.type !== 'machine' && actor.type !== 'admin') {
-		throw new VisibleError(
-			'forbidden',
-			ErrorCodes.Permission.INSUFFICIENT_PERMISSIONS,
-			'Machine or admin credentials required'
-		);
-	}
-	return next();
-};
-
-export const adminOnly: MiddlewareHandler = async (_, next) => {
-	const actor = Actor.use();
-	if (actor.type !== 'admin') {
-		throw new VisibleError(
-			'forbidden',
-			ErrorCodes.Permission.INSUFFICIENT_PERMISSIONS,
-			'Admin access required'
-		);
-	}
-	return next();
-};