Add native Slack notification module

[hostarts]

Summary

Adds first-class Slack delivery alongside the existing email path. Until now, posting backup results to Slack required wrapping vmbackup.sh in ExecStopPost or a cron post-hook, which only sees the systemd $SERVICE_RESULT/$EXIT_STATUS and has to re-derive session totals from the DB out-of-band. With this PR, Slack posts use the same sqlite_query_session_summary row the email module uses, fire from the same four call sites (so failure-path notifications still go out when a run aborts before normal end-of-main), and stay enabled/disabled independently of email.

What changed

• modules/slack_notification_module.sh (new): load_slack_config() + send_slack_notification() mirroring load_email_config() / send_backup_report(). Posts a coloured attachment with VM totals (ok / failed / skipped / excluded), total bytes, duration, and instance name. JSON is built inline; only runtime dependency is curl.
• config/template/slack.conf (new): SLACK_ENABLED, SLACK_WEBHOOK_URL, SLACK_HOSTNAME, SLACK_TITLE_PREFIX, SLACK_ON_SUCCESS, SLACK_ON_FAILURE, SLACK_TIMEOUT. Independent flags so Slack-only / email-only / both are all valid combinations.
• vmbackup.sh: wired Slack into the same four sites as email — cleanup_on_exit (pre-flight aborts), handle_sigterm (systemd timeout / manual stop), the replicate-only mode end, and the normal session end in main. _SLACK_SENT guard prevents duplicate posts the same way _EMAIL_SENT does.
• CHANGELOG.md: [Unreleased] entry under ### Added.

Design notes

• Mirrors email rather than replacing it. Same enable/condition gates (*_ON_SUCCESS/*_ON_FAILURE), same call sites, same data source. Operators who already understand the email config don't have to learn a new mental model.
• No new dependencies beyond curl. No jq. JSON is escaped with a small in-module helper (_slack_json_escape) so the module can drop into an existing AlmaLinux/Debian install with no extra packages.
• Falls back gracefully. If SQLite isn't available, the post still goes out with zeroed totals — the operator still gets the success/failure signal.
• Default-off. SLACK_ENABLED=\"no\" in the template; no behaviour change for existing installs.

Test plan

• bash -n vmbackup.sh + bash -n modules/slack_notification_module.sh pass.
• load_slack_config round-trips a minimal slack.conf (smoke-tested locally).
• _slack_format_bytes produces TiB/GiB/MiB/KiB/B with correct boundaries.
• Verified the four vmbackup.sh integration sites match the four corresponding email sites (grep confirms 4×send_slack_notification next to 4×send_backup_report).
• End-to-end run on real backup hosts (verified in operator environment, not part of this PR).

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