Notify on every run (Discord)

notify.py, built around an event model so new events are cheap to add: notify(event, payload) with event in {backup_success, backup_degraded, backup_failure, drill_success, drill_failure, ...}.

• Discord webhook (BUR_DISCORD_WEBHOOK) subscribes to every event, so a message lands on every run, success and failure, with the per-database summary (which databases ran, the dumped and pushed booleans, sizes, a total, and any error). This is a channel webhook, not a bot.
• Severity follows the state-file semantics: backup_success is info, backup_degraded is a warning, backup_failure is an error. Discord gets all three. --quiet suppresses the success message for ad-hoc --only runs, but failures always notify.

Acceptance

• every completed run posts to Discord (success, degraded, and failure) with a per-database summary, and drill results post too.
• new event types can be added without touching existing senders; the channel is optional, and its absence falls back to log-only.