feat: add GET/PUT /v3/settings server settings API

GET returns all settings sections except the deprecated
VirtualBox/VMware ones (Server.Audit privilege); secrets are
masked and Controller.jwt_secret_key is excluded entirely.
PUT applies a partial update (Server.Modify privilege): masked
or empty secrets mean unchanged, null removes the option, the
response carries the new values plus restart_required, and a
settings.updated notification is emitted. New
Server.Audit/Server.Modify privileges are seeded into the
Administrator role at table creation.

gns3server/db/models/privileges.py

@@ -238,6 +238,14 @@ def create_default_roles(target, connection, **kw):
         {
             "description": "Update an LLM model configuration",
             "name": "LLMConfig.Modify"
+        },
+        {
+            "description": "View server settings",
+            "name": "Server.Audit"
+        },
+        {
+            "description": "Update server settings",
+            "name": "Server.Modify"
         }
     ]