fix(api): a misshapen id is bad input, not a server fault

Ids are stored in a fixed-width column, so an overlong one is refused by
Postgres rather than simply matching nothing. That refusal is not a foreign-key
violation, so it fell through to the global error boundary and reached the
caller as a 500 — telling a host to retry something that can never succeed.
Measured: a 44-character user id returned 500, where an absent but well-formed
one correctly returned 404.

`Identifier.schema` is the natural place for the check and had no callers yet,
so it now asserts the exact width an id has as well as its prefix — including
the separator, without which `usrsomething` reads as a user id. The enrolment
schema uses it for both foreign keys, so the refusal happens where the input
arrives and names the field.

Also index `steam_enrolment.user_id`. The primary key begins with the machine,
which answers what one host holds and nothing else, so neither of the two
things that read by user alone can use it: the cascade behind deleting a user,
and asking which hosts hold a token for one person. The table's migration has
not been released, so this is folded into it rather than following it with a
correction.

packages/core/src/id.ts

@@ -27,8 +27,24 @@ export namespace Identifier {
 		refreshToken: 'rft'
 	} as const;
 
+	/**
+	 * An id as this control plane issues them: the right prefix, and the exact
+	 * width the column has.
+	 *
+	 * The width is the half that matters at an API boundary. Ids are stored in
+	 * a fixed-width column, so an overlong string is *refused by the database*
+	 * rather than simply not matching anything — which surfaces to the caller
+	 * as a server fault instead of the validation error it actually is.
+	 * Checking it where the input arrives is what keeps the two apart.
+	 *
+	 * The separator is part of the prefix check for the same reason: without
+	 * it, `usrsomething` reads as a user id.
+	 */
 	export function schema(prefix: keyof typeof prefixes) {
-		return z.string().startsWith(prefixes[prefix]);
+		return z
+			.string()
+			.startsWith(`${prefixes[prefix]}_`)
+			.length(prefixes[prefix].length + 1 + LENGTH);
 	}
 
 	const LENGTH = 26;