v0.16.0 — more notification channels

Notifications are no longer Home-Assistant-only. notify() now fans every alert out to all configured channels:

• Home Assistant push (as before) · Pushover · Pushbullet · Discord (rich embed) · Telegram · ntfy

Set any you like on the Settings page (or in .env) — each fires only when its own keys are present, and one failing channel never blocks the others. The tap-to-open link is reused on every channel, and the Test button sends to all of them at once. 59 tests. Closes #28.