Add commit URL to Slack notifications with link and backtick formatting

- Add commit_url column to webhook_events and deployments tables
- Extract commit URL from webhook payload (from commit object or repo URL)
- Format Slack messages with backticks for branch and commit SHA
- Link commit SHA to the actual commit URL on the git server
- Keep plain text format for ntfy notifications

internal/database/migrations/004_add_commit_url.sql

@@ -0,0 +1,3 @@
+-- Add commit_url column to webhook_events and deployments tables
+ALTER TABLE webhook_events ADD COLUMN commit_url TEXT;
+ALTER TABLE deployments ADD COLUMN commit_url TEXT;