Replace the custom polling loop in the Slack sender with the
go.gearno.de/kit/worker framework, matching the email sending worker.
Add a processing_started_at column to slack_messages so claims are safe
under the worker's claim/process model, with stale recovery for crashed
in-flight sends. Rename sender.go to worker.go to match the kit worker
file naming convention.
Signed-off-by: Sacha Al Himdani <sacha@probo.com>
f7e93cdec5