Compress post content before caching to Redis, decompress when fetching

This PR aims to reduce memory pressure on Redis, given that we have some posts with content exceeding 1 MB. Given that any amount of posts can be in our cache based on current activity, we have a chance of bloating Redis with high-content posts.

This PR compresses Post content before caching it, also adding a compressed- plaintext prefix. This prefix is used to ensure that we only decompress post content that is compressed, so that older cache entries are still available. We should be able to remove that prefix after ~30 days