feat(rust): Backpacks — staff and player views, the Bigger backpack step (PLAN_REDESIGNS §11.3)

The site half of §11.3.2, on protocol 14:

- `rust.backpack.size` ("Bigger backpack"): a kit reward with a different
  permission (D337) — one rust_perm_run_grants row per website user with
  backpacks.size.N, no kit, no credit; the perm mirror pushes it and the run's
  revert deletes it. risk change, reversible ledger, a new `rust.backpacks`
  budget, rust.kit.entitle's recipient modes including `linked`. Refused with
  "Backpacks is not installed on this server" in the dry run too (D199), and a
  size the server does not offer is refused by name. Its message is never
  silent: empty is the take-back warning (D338).
- `rust.options.backpacksizes`: each server's sizes above its smallest, from
  the last hello, grouped by server.
- GET /player/rust/backpacks (own links only, 20 reads a minute) and
  GET /admin/users/:id/rust/backpacks (that user's links only). Live, never
  stored; a stranger's id is a 404 and never reaches the game.
- Client: a shared Backpack panel (pick server and account, open, read once)
  on the account page and the admin user page; the grid draws pages of
  pageSize, each slot named by the game (D340, no icons), blueprints by what
  they make (D341), overflow marked. Backpacks joins the server's mod line.

Server 548/548 (13 new in test/backpacks.test.js), client 79/79 (5 new),
routes.manifest regenerated against the pinned core (89 routes), swagger
fragment regenerated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/router/admin/usersRust.router.js

@@ -30,7 +30,7 @@
 const core = require('../../core')
 
 const express = core.express
-const { body, param } = core.validator
+const { body, param, query } = core.validator
 
 const usersRust = require('./usersRust.controller')
 const { validate } = core.middleware
@@ -171,4 +171,29 @@ usersRustRouter.post(
   usersRust.adjustEconomics,
 )
 
+// ── Backpacks (PLAN_REDESIGNS §11.3.2): what a person carries, read live ──
+//
+// Read only (R2): there is no write beside it, here or in the game. Each open
+// of the panel is one read; nothing is stored.
+
+usersRustRouter.get(
+  '/rust/backpacks',
+  // #swagger.tags = ['Admin · Users']
+  // #swagger.summary = 'A user’s backpack on one server (admin only)'
+  // #swagger.description = 'One of this user’s linked Steam accounts’ Backpacks backpack on one server, read live from the game and never stored. The same shape as GET /player/rust/backpacks. Staff see the overflow too; only the owner’s own open of the backpack moves it.'
+  // #swagger.security = [{ "cookieAuth": [] }, { "bearerAuth": [] }]
+  // #swagger.parameters['id'] = { in: 'path', required: true, schema: { type: 'integer' }, description: 'User id.' }
+  // #swagger.parameters['serverId'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'The server.' }
+  // #swagger.parameters['steamId'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'One of this user’s linked Steam ids.' }
+  /* #swagger.responses[200] = { description: 'The backpack', content: { "application/json": { schema: { $ref: "#/components/schemas/RustBackpack" } } } } */
+  /* #swagger.responses[404] = { description: 'No such server, or that Steam account is not linked to this user', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  /* #swagger.responses[409] = { description: 'Backpacks is not installed on that server', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  /* #swagger.responses[503] = { description: 'The server could not be reached', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  param('id').isInt(),
+  query('serverId').isString().isLength({ min: 1, max: 64 }),
+  query('steamId').isString().matches(STEAM_ID_RE),
+  validate,
+  usersRust.readBackpack,
+)
+
 module.exports = usersRustRouter