Migrate Slack sender to kit worker

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>

pkg/coredata/migrations/20260623T100000Z.sql

@@ -0,0 +1,15 @@
+-- Copyright (c) 2026 Probo Inc <hello@probo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE slack_messages ADD COLUMN processing_started_at TIMESTAMP WITH TIME ZONE;