Document token permission required for load capture (loadcap) API

## Summary

Document which token permission the load capture (`loadcap`) API and
CLI require on InfluxDB 3 Enterprise. This was left untested while
verifying the feature for the load capture guide.

## Context

The load capture guide (`content/influxdb3/enterprise/admin/load-capture.md`)
and the API reference (issue #7466, PR #7476) were verified against
InfluxDB 3 Enterprise 3.10.2. Verification used an **admin token** for
all `loadcap` requests, which worked. Whether a database-scoped or other
non-admin permission token can run `loadcap` was **not tested**.

The guide currently says "The examples below use an admin token" without
stating whether admin is required.

## What to verify

Against a `--mode query` Enterprise node, for each `loadcap` operation
(`start`, `list`, `files`, `preview`, `download`, `delete`):

- Does an admin token work? (confirmed for `start`/`list`/`preview`/`download`/`delete`)
- Does a database-scoped permission token work, or return `403`?
- Is there a specific permission/action that grants `loadcap` access?

## Acceptance

- The load capture guide states the required token permission.
- The API reference documents the required permission for the `loadcap`
endpoints.

## Notes

- Load capture is Enterprise-only and available only on a node running
`--mode query`.
- Related: #7466, #7476.

Start with content/influxdb3/enterprise/admin/load-capture.md and the loadcap API reference from issues #7466 and #7476. Against an InfluxDB 3 Enterprise 3.10.2 node running --mode query, test admin and database-scoped tokens for start, list, files, preview, download, and delete. Done means both the guide and API reference state the required token permission.

Written by the indexing model from the issue text.