Rename "Episode" class to "Chapter" or "Page"

During user testing for the new interface, users expressed confusion about the term "Episode", referring to individual chapters/units/files of a lesson. The terminology of "episode" is intended to reflect that each one should be no more than 15 to 45 minutes long (about the length of an episode in a television series), but that aspect is very nuanced. A better term is "chapter".

We should reconsider naming the class that contains each chapter/unit/file/page an "Episode" and call it either "Chapter" or "Page" to make it more semantically correct.

Start by locating the Episode class and every reference to it in the repository. The issue does not name files or tests and leaves the replacement term undecided, so first resolve whether Chapter or Page is intended. Done would require a consistent rename across the affected project surface.

Written by the indexing model from the issue text.