Why should I save both markdown and the converted HTML in my database?
Your argument is flawed.

You are concerning yourselves with doubling a 15kB per article  × 10k rows = 150MB table. (Excluding images)

And, you will be accessing individual rows via a PK.  You can have a trillion rows and the database won't care.  10k rows is tiny.

Additionally, the database is already going to be inflated due to Revisions and Text Indexing .  Adding the HTML version shouldn't hurt you.  Not even at 10M rows.

Oh, if the author is editing the Markdown version, you better record what she/he wrote.  (Ie always convert from the author's format to the printed format; never the other way around)