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/sidecarClient.js

@@ -474,6 +474,13 @@ const economicsBalances = (server, steamIds) =>
 /** Staff: `{ key, steamId, op, amount }`, op deposit · withdraw · set. `economics.adjusted` has before and after. */
 const economicsAdjust = (server, body) => request(server, '/economics/adjust', { method: 'POST', body })
 
+/**
+ * One player's backpack (protocol 14, PLAN_REDESIGNS.md §11.3.2), read live.
+ * `backpacks.contents` has `exists`, and with a backpack its capacity, page
+ * size and stacks; `backpacks.error` has a reason. Nothing is written.
+ */
+const backpacksRead = (server, steamId) => request(server, '/backpacks/read', { method: 'POST', body: { steamId } })
+
 /**
  * Say one line in the server's chat. `chat.ok` carries `said: false` when the
  * key was said in the last ten minutes — a retry, answered and not repeated.
@@ -566,6 +573,7 @@ module.exports = {
   economicsPay,
   economicsBalances,
   economicsAdjust,
+  backpacksRead,
   chat,
   titles,
   mapInfo,