65 post caption store new caption in the db for performance

[yshmarov]

Description

before, each time a post caption was displayed, it had to get parsed by the mentions & hashtags helper.

now, when InstaPosts are imported/updated by the InstaMediaService, the processed_caption will be the one with mentions & hashtags => no need to run the helper each time => cheaper runtime

How to test

Self-check

• [x] self-reviewed
• [x] added tests
• [x] focused (related only to the ticket scope)

Visual Changes

Before

After