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/test/eventModuleContract.test.js

@@ -112,7 +112,7 @@ test('the version a module declares against satisfies ^1.10.0', () => {
   // 1.10.0 loading — Module-uo declares exactly `^1.10.0` — and this is the
   // assertion that says it does. The number itself is pinned in version.js.
   assert.ok(semver.satisfies(MODULE_API_VERSION, '^1.10.0'), MODULE_API_VERSION)
-  assert.equal(MODULE_API_VERSION, '1.16.0')
+  assert.equal(MODULE_API_VERSION, '1.17.0')
 })
 
 test('a module registers actions, budgets, leases and option sources', () => {