Post subscription notification feature

Howorequested to merge
feature/subscribe-post into develop

Allow users to subscribe to posts/comments via custom_json and receive notifications for all new descendants (children, grandchildren, etc.).

Features:

  • New custom_json operation: subscribe_post with subscribe/unsubscribe commands
  • New notification type: post_subscription_reply (type_id=18)
  • Recursive CTE walks up parent chain to find all ancestor subscribers
  • New API endpoint: bridge.get_post_subscriptions to list subscriptions
  • Notifications respect mute relationships and reputation scores

Schema changes:

  • New table: hive_post_subscriptions (account_id, post_id, created_at, block_num)

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