fix: podcast slug 404 handling

[JoeKarow]

Linked Issue

Fixes #622

Description

If an invalid episode slug was requested via /podcast/[slug], a 500 error was thrown instead of a 404.

Files that revolve around the podcasty stuff were also converted to TypeScript.

Methodology

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[danieltott]

@JoeKarow thank you so much for this - this is great 💪