Jordan Williams authored Dec 13, 2020 and Jordan Petridis committed Dec 24, 2020

Previously, cached podcast cover art was never invalidated.
When podcasts updated cover art, it would not be updated locally.
To correct this, a time-based caching mechanism has been introduced.
This invalidates cached images after a specific amount of time.
This has been set to 4 weeks.
Once a podcast cover has been stale for 1 month, it is re-downloaded.
To get updates earlier, a hash-based mechanism will be implemented.
The time-based mechanism is mostly a fallback to guarantee updates.

One column is added to the shows table, the timestamp image_cached.
A database migration has been included as part of this commit.

Testing has yet to be implemented for this functionality.