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/routes/steam.ts

@@ -79,7 +79,7 @@ export namespace SteamApi {
 			describeRoute({
 				tags: ['Steam'],
 				summary: 'Link a Steam account',
-				description: 'Link a Steam account to a user (admin) or yourself (user)',
+				description: 'Link a Steam account to the calling user.',
 				responses: {
 					200: {
 						content: {
@@ -113,13 +113,6 @@ export namespace SteamApi {
 						description: 'Steam ID to link',
 						example: '76561197960287930'
 					}),
-					userId: z
-						.string()
-						.optional()
-						.meta({
-							description: 'User ID to link to (admin only; omitted when linking your own account)',
-							example: Examples.Id('user')
-						}),
 					profile: z
 						.record(z.string(), z.unknown())
 						.optional()
@@ -131,20 +124,14 @@ export namespace SteamApi {
 			),
 			async (c) => {
 				const body = c.req.valid('json');
-				const actor = Actor.use();
-
-				if (body.userId && actor.type !== 'admin') {
-					throw new VisibleError(
-						'forbidden',
-						ErrorCodes.Permission.INSUFFICIENT_PERMISSIONS,
-						'Only admin can link a Steam account for another user'
-					);
-				}
 
+				// Linking is always for the caller. It once accepted a `userId`,
+				// which meant one credential could attach a Steam account to any
+				// user \u2014 and a linked account is how a library is reached.
 				const linkedAccountID = await Steam.link({
 					steamId: body.steamId,
 					profile: body.profile,
-					userId: body.userId
+					userId: Actor.userID
 				});
 				return c.json({
 					data: { linkedAccountId: linkedAccountID, steamId: body.steamId }