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/swagger/swagger-output.json

@@ -7211,7 +7211,7 @@
           "Admin · Modules"
         ],
         "summary": "Install or upgrade a module from a release install-manifest URL",
-        "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.",
+        "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.",
         "responses": {
           "201": {
             "description": "Installed — restart to mount it",
@@ -7234,6 +7234,16 @@
               }
             }
           },
+          "409": {
+            "description": "A different module is already installed; a site runs one module",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
           "500": {
             "description": "Internal Server Error"
           },