[OPE-167] Fix Slack reaction handler: refresh card and post confirmation after ack/resolve

[singret]

Summary

• After a ✅ or 🔴 reaction on the incident card updates status, the Slack card now refreshes to reflect the new status/buttons (was staying stale)
• Posts a public <@user> acknowledged/resolved INC-N via reaction message so the team sees who acted — mirrors the existing button handler behaviour
• Adds a missing early return in the error path so the confirmation isn't posted when the DB update fails

Root cause

handleReactionAdded called UpdateIncidentStatus then did nothing after — no card refresh, no channel message. The button handler (handleStatusButton) did both. This brings reactions into parity.

Test plan

• Add ✅ reaction to incident card in Slack → incident moves to Acknowledged in Regen UI, card updates, confirmation message appears in channel
• Add 🔴 reaction to incident card in Slack → incident moves to Resolved in Regen UI, card updates, confirmation message appears in channel
• Reaction on a non-incident message → no action taken
• Reaction on already-resolved incident → gracefully ignored (existing transition guard)

Note: Requires Slack app Event Subscriptions to be enabled in the app dashboard (message.channels, app_mention, reaction_added) and the app reinstalled. See OPE-169 for the manifest fix.