docs: roll back account_id query param on GET /api/chats/{id}/messages

The override param is redundant for this endpoint: the chat is fully
identified by the path {id}, and selectRoom already resolves room.account_id,
so admin access should be handled by an admin-aware access check in
validateChatAccess (RECOUP_ORG bypass), not by an extra query param the
caller would have to supply. Keeps the DELETE /api/artists/{id} body
override, where the param is load-bearing (it names whose ownership link
to drop).

Refs recoupable/chat#1811

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sweetmantech

api-reference/openapi/research.json

@@ -343,16 +343,6 @@
               "type": "string",
               "format": "uuid"
             }
-          },
-          {
-            "name": "account_id",
-            "in": "query",
-            "description": "Read messages in the context of a specific account. Only applicable when the authenticated account has access to multiple accounts via organization membership or Recoup admin access.",
-            "required": false,
-            "schema": {
-              "type": "string",
-              "format": "uuid"
-            }
           }
         ],
         "responses": {
@@ -367,7 +357,7 @@
             }
           },
           "400": {
-            "description": "Bad request - invalid chat id or account_id",
+            "description": "Bad request - invalid chat id",
             "content": {
               "application/json": {
                 "schema": {