Fix Patch OpenAPI spec

[imobachgs]

Problem

• The specification for the /PATCH config action is incomplete.
• According to doc/http_api.md: missing PATCH /questions schema and GET /questions response description #3355, the doc/questions.md file is outdated.

Solution

      "Patch": {
        "type": "object",
        "description": "Patch for the config.",
        "properties": {
          "update": {
            "oneOf": [
              {
                "type": "null"
              },
              {
                "$ref": "#/components/schemas/Config",
                "description": "Update for the current config."
              }
            ]
          }
        }
      },

This change was implemented by Claude Code.

Testing

• Tested manually

[mchf]

LGTM