User subscriptions

[psilabs-dev]

This could be a cronjob, or a separate microservice that handles scheduled tasks...?

POST /api/subscribe/user/{pixiv_user_id} # add a subscription to a user (with optional metadata)
GET /api/subscribe/user # get all user subscriptions and subscription metadata
DELETE /api/subscribe/user/{pixiv_user_id} # delete subscription by user ID (deletes metadata)

Then, PixivUtil server will just store active subscriptions and serve them. A cron job will be used to trigger download jobs based on these subscriptions.