Add Discord account registration monitor (#7013)

A read-only cron script that posts new Servatrice account registrations to a
Discord channel via webhook. Dedups by an auto-increment id high-water-mark
(single-integer state, no duplicates, nothing missed across downtime). Reads DB
credentials and the webhook from a servatrice-style ini via --config.

servatrice/scripts/account_monitor/.gitignore

@@ -0,0 +1,5 @@
+# Local state - never commit these
+state.json
+state.json.tmp
+venv/
+__pycache__/