fix(modules): a site runs one module; the installer refuses a second

A site is one game, and the module contract already has singletons that
assume it. registerTeamProvider holds one value per deployment, and a
second module registering one fails that module's whole load. The loader
scans alphabetically, so installing module-rust (which gains a Team
provider in its phase 9) beside module-uo would have taken uo down, not
rust.

install() now refuses, with 409 and before the artifact is downloaded,
any install whose id differs from a module already on the volume. An
upgrade of the installed module is still accepted; to change game,
remove the module first. Both install surfaces share this path, so a
MODULES declaration naming two modules installs the first and reports
the second as refused without failing the boot.

"Installed" means what the loader would scan: a directory named with a
module id that holds a module.json. An install's scratch directory and a
swap's aside copy do not count.

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

server/src/router/v1/admin/modules.router.js

@@ -41,11 +41,12 @@ modulesRouter.post(
   '/',
   // #swagger.tags = ['Admin · Modules']
   // #swagger.summary = 'Install or upgrade a module from a release install-manifest URL'
-  // #swagger.description = 'Downloads the artifact the manifest names, verifies its sha256, inspects the archive in full and unpacks it onto the modules volume. The module mounts on the next restart.'
+  // #swagger.description = 'Downloads the artifact the manifest names, verifies its sha256, inspects the archive in full and unpacks it onto the modules volume. The module mounts on the next restart. A site runs ONE module: installing a module other than the one already on the volume is refused with 409 before anything is downloaded, and only an upgrade of the installed module is accepted.'
   // #swagger.security = [{ "cookieAuth": [] }, { "bearerAuth": [] }]
   /* #swagger.requestBody = { required: true, content: { "application/json": { schema: { type: "object", required: ["url"], properties: { url: { type: "string", description: "https URL of the release install manifest, on an allowed host" } } } } } } */
   /* #swagger.responses[201] = { description: 'Installed — restart to mount it', content: { "application/json": { schema: { type: "object", additionalProperties: true } } } } */
   /* #swagger.responses[400] = { description: 'The URL, the manifest, the hash or the archive was refused', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  /* #swagger.responses[409] = { description: 'A different module is already installed; a site runs one module', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
   /* #swagger.responses[502] = { description: 'The source host could not be reached or answered badly', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
   adminOnly,
   body('url').isString().trim().isLength({ min: 1, max: 2048 }),