no issue
- cleans up unused tables `emails.{meta,stats}`
- adds timestamp columns `email_recipients.{delivered_at,opened_at,failed_at}` that can be used for event timelines and basic stats aggregation
- indexed because we want to sort by these columns to find the "latest event" when limiting Mailgun events API requests
- adds aggregated stats columns `emails.{delivered_count,opened_count,failed_count}`
- adds a composite index on `email_recipients.[email_id,member_email]` to dramatically speed up `email_recipient` update queries when processing events
- modifies the db initialisation to support an `'@@INDEXES@@'` key in table schema definition for composite indexes
