Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

docs/web-ui.md

@@ -5,7 +5,7 @@ _What the web UI offers, how sign-in and sessions work, and the Team and Admin t
 The server serves a web UI at `/`: the incident queue, each incident's alerts
 and timeline with every action (acknowledge, assign, snooze, note, resolve,
 archive), who is on call, the alert feed, and an *Account* tab for your own
-password and the ntfy topic your pages go to. It is built for a phone first. On a phone
+password and the notification channels your pages go to. It is built for a phone first. On a phone
 it navigates through a hamburger menu and has a sticky action bar, it follows the
 system's dark mode, and it can be added to the home screen. From 900px wide it switches
 to a sidebar with the queue and the incident side by side. The Stats page shows