feat(auth): sign in with Steam (MODULE_API 1.17.0)

Steam is a built-in provider beside Google and Discord (STEAM_SIGNIN.md,
D392-D396). OpenID 2.0: the assertion is verified server to server with
check_authentication, return_to must match byte for byte, and the claimed id
must be a 17-digit SteamID64. A Steam Web API key is required to enable it
(D394); persona name and avatar are stored on the identity and refreshed at
each sign-in, and a failed lookup never blocks one. Steam never signs anybody
up (D395), on the web or the mobile bridge. Several Steam accounts may be
linked; DELETE /auth/me/account/identities/:provider takes ?id= to unlink one.

MODULE_API 1.17.0: ctx.identities.list, api.onIdentityLinking (refuse-only,
fails closed, module text never shown) and api.onIdentityLinked. No unlink
hook (D393). Schema additive; OpenAPI, routes and engagement manifests
regenerated with no modules installed.

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

server/src/auth/identityHooks.js

@@ -0,0 +1,97 @@
+// ── Modules and identity links (MODULE_API 1.17.0) ─────────────────────────
+//
+// Core owns linking an external sign-in identity to an account. Modules take part
+// in exactly two places (STEAM_SIGNIN.md §4):
+//
+//   checkLink()   before the identity is written. Each module that registered
+//                 `onIdentityLinking` may refuse. Rust uses it for D396: a Steam
+//                 id that is already another website account's Rust link.
+//   notifyLinked() after it is written. Rust uses it for D392: the Steam account
+//                 becomes a linked Rust player.
+//
+// A refusal can only stop a link. It runs inside the link flow of a user who is
+// already signed in, so nothing here can mint a session or let through a link
+// core would refuse.
+//
+// What a module says is never shown to the player: `reason: 'in_use'` reads as
+// the same "already linked to another user" core says itself, and anything else
+// as a plain refusal. A module's own words go to the log. Putting module text in
+// a redirect URL would let anybody craft a link that prints a sentence of their
+// choosing on our account page.
+
+const loader = require('../modules/loader')
+const log = require('../utils/logger')('sso')
+
+const HOOK_TIMEOUT_MS = 5_000
+
+function withTimeout(fn, ms) {
+  let timer
+  const deadline = new Promise((_, reject) => {
+    timer = setTimeout(() => reject(new Error(`no answer within ${ms} ms`)), ms)
+  })
+  return Promise.race([Promise.resolve().then(fn), deadline]).finally(() => clearTimeout(timer))
+}
+
+/**
+ * Ask every started module whether this link may be made.
+ *
+ * **Fails closed.** A module that throws or does not answer in time refuses the
+ * link: the hook exists to stop a link that must not happen, and a module that
+ * could not look is a module that could not say it was safe. The player is told
+ * to try again; a link is never the urgent half of anything.
+ *
+ * @param {{userId: number, provider: string, subject: string}} link
+ * @returns {Promise<{ok: true} | {ok: false, reason: 'in_use' | 'refused', module: string}>}
+ */
+async function checkLink(link) {
+  const frozen = Object.freeze({ userId: link.userId, provider: link.provider, subject: link.subject })
+  for (const { id, hook } of loader.identityHooks('onIdentityLinking')) {
+    let answer
+    try {
+      answer = await withTimeout(() => hook(frozen), HOOK_TIMEOUT_MS)
+    } catch (err) {
+      log.error('identity link check failed; refusing the link', { module: id, provider: frozen.provider, message: err.message })
+      return { ok: false, reason: 'refused', module: id }
+    }
+    if (answer && answer.ok === false) {
+      const reason = answer.reason === 'in_use' ? 'in_use' : 'refused'
+      log.info('identity link refused by a module', {
+        module: id,
+        provider: frozen.provider,
+        userId: frozen.userId,
+        reason,
+        message: typeof answer.message === 'string' ? answer.message.slice(0, 200) : undefined,
+      })
+      return { ok: false, reason, module: id }
+    }
+  }
+  return { ok: true }
+}
+
+/**
+ * Tell every started module a link was written.
+ *
+ * Awaited (bounded) so the page the player lands on already shows what a module
+ * made of it — the Rust page listing the Steam account they just linked — but a
+ * failure never reaches the link: the identity is written, and that is core's
+ * answer. A module that missed it is logged.
+ *
+ * @param {{userId: number, provider: string, subject: string, displayName?: string|null}} link
+ */
+async function notifyLinked(link) {
+  const frozen = Object.freeze({
+    userId: link.userId,
+    provider: link.provider,
+    subject: link.subject,
+    displayName: link.displayName || null,
+  })
+  await Promise.all(
+    loader.identityHooks('onIdentityLinked').map(({ id, hook }) =>
+      withTimeout(() => hook(frozen), HOOK_TIMEOUT_MS).catch((err) => {
+        log.error('identity linked hook failed', { module: id, provider: frozen.provider, message: err.message })
+      }),
+    ),
+  )
+}
+
+module.exports = { checkLink, notifyLinked, HOOK_TIMEOUT_MS }