Add article: Normalize the Loudness of a Folder of Videos with PowerShell and FFmpeg (#501)

This adds an article, "Normalize the Loudness of a Folder of Videos with
PowerShell and FFmpeg", and my author profile (made with
`tools/new-author.ps1`).

The article builds an advanced function that measures every video in a
folder with FFmpeg's two-pass `loudnorm` filter. It normalizes the files
that miss the target in `ForEach-Object -Parallel`, supports `-WhatIf`,
skips files that are already within tolerance, and re-measures what it
writes. It also covers the problems I ran into: reading the `loudnorm`
report from stderr, `$using:` and runspace scope in `-Parallel`, a colon
after a variable inside a string, and when `loudnorm` falls back from
linear to dynamic mode.

I drafted this with Claude Code; every command in it was run on
PowerShell 7.6.9 with FFmpeg 9.0.2, and the output shown is from those
runs. PSScriptAnalyzer reports no findings for the function, and the
site builds with Hugo 0.155.1 with `validate:articles` passing.

It's dated for today; happy to move it to whatever date suits the
schedule.

Author: markksantos