Refactor admin procedures into separate files

Extract admin procedures from router.ts into dedicated files under
procedures/admin/ with consolidated exports via _routes.ts. Adds shared
helper functions for response transformation and includes race condition
fixes via transaction-scoped existence checks.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api-server/src/procedures/admin/users/list.ts

@@ -0,0 +1,14 @@
+/**
+ * admin.users.list - List all users
+ */
+
+import { authMiddleware, os, superuserMiddleware } from "../../base.js";
+import { toUserResponse } from "../helpers.js";
+
+export const adminUsersList = os.admin.users.list
+  .use(authMiddleware)
+  .use(superuserMiddleware)
+  .handler(async ({ context }) => {
+    const users = await context.db.selectFrom("users").selectAll().execute();
+    return users.map(toUserResponse);
+  });