Bug: runJob computed since = sub.last_digest_at, fetched events with
received_at > since, sent the digest (slow), then deleted ALL events
with received_at > since. Any /notify event that arrived between the
fetch and the delete was also received_at > since, so it was deleted
without ever being sent in a digest.
Two changes:
1. Delete by exact event IDs (src/database.ts, src/worker/email.ts):
Added deleteEventsByIds(subscriptionId, eventIds) which deletes
only the events that were actually fetched + sent. The old
timestamp-based delete is retained but no longer called from runJob.
2. Re-fetch subscription on each cron tick (src/worker/email.ts):
createJob's closure captured the original sub, so sub.last_digest_at
stayed stale in memory. Every subsequent tick recomputed since from
the old value, re-fetching and re-sending duplicate events. Now each
tick re-fetches the subscription from the DB via getSubscriptionById
before calling runJob.
Fixes bead mailship-091
