Show notes: Not always available for downloaded episodes

### Description

We've had a user report that show notes aren't working for them on downloaded episodes when they have no internet connection. It can be replicated for them when in airplane mode.

Internal Discussion: p1728386945742059-slack-C02A333D8LQ

### Step-by-step reproduction instructions

We could not replicate, but @SergioEstevao and @danielebogo have some theories:

> could it be because we store it in nscache and for some reasons the system cleans it?
> So it looks he downloaded the episodes beforehand, and a while ago, so meanwhile while the episodes are saved in a folder controlled by us, the episode details are in a system cache that can be deleted

### Screenshots or screen recording

Internal Link of the video: p1728568967947419/1728386945.742059-slack-C02A333D8LQ


### Did you search for existing bug reports?

- [x] I have searched for existing bug reports.

### Device, Operating system, and Pocket Casts app version

Version 7.73 (7.73.0.6), iOS 15.8.3, iPhone SE model MP822X/A.

Start by reproducing the report with a downloaded episode in airplane mode, using the iOS 15.8.3 and app version 7.73 details provided. Investigate how downloaded episode details and show notes are retained offline, including the reported NSCache theory. Done means show notes remain available for downloaded episodes without an internet connection, including after the system may clear its cache.

Written by the indexing model from the issue text.