🐛(backend) disable pagination for thread messages

Long threads (more than 20 messages) only displayed the first 20 messages
because the MessageViewSet used the default PAGE_SIZE of 20. The frontend
doesn't handle pagination for messages, so additional messages were silently
hidden from users.

This fix disables pagination for the messages endpoint since threads typically
don't have enough messages to require pagination.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/core/api/openapi.json

@@ -3859,17 +3859,6 @@
             "get": {
                 "operationId": "messages_list",
                 "description": "ViewSet for Message model.",
-                "parameters": [
-                    {
-                        "name": "page",
-                        "required": false,
-                        "in": "query",
-                        "description": "A page number within the paginated result set.",
-                        "schema": {
-                            "type": "integer"
-                        }
-                    }
-                ],
                 "tags": [
                     "messages"
                 ],
@@ -3883,7 +3872,10 @@
                         "content": {
                             "application/json": {
                                 "schema": {
-                                    "$ref": "#/components/schemas/PaginatedMessageList"
+                                    "type": "array",
+                                    "items": {
+                                        "$ref": "#/components/schemas/Message"
+                                    }
                                 }
                             }
                         },
@@ -6736,37 +6728,6 @@
                     }
                 }
             },
-            "PaginatedMessageList": {
-                "type": "object",
-                "required": [
-                    "count",
-                    "results"
-                ],
-                "properties": {
-                    "count": {
-                        "type": "integer",
-                        "example": 123
-                    },
-                    "next": {
-                        "type": "string",
-                        "nullable": true,
-                        "format": "uri",
-                        "example": "http://api.example.org/accounts/?page=4"
-                    },
-                    "previous": {
-                        "type": "string",
-                        "nullable": true,
-                        "format": "uri",
-                        "example": "http://api.example.org/accounts/?page=2"
-                    },
-                    "results": {
-                        "type": "array",
-                        "items": {
-                            "$ref": "#/components/schemas/Message"
-                        }
-                    }
-                }
-            },
             "PaginatedThreadAccessList": {
                 "type": "object",
                 "required": [