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

internal/api/middleware.go

@@ -123,8 +123,8 @@ func AdminOnly(next http.Handler) http.Handler {
 }
 
 // requireSelfOrAdmin guards the endpoints that are self-service for your own
-// account and administration for anybody else's: your password, your ntfy
-// topic, your API keys. Reports whether the request may proceed, and answers it
+// account and administration for anybody else's: your password, your notification
+// channels, your API keys. Reports whether the request may proceed, and answers it
 // if not.
 //
 // An API key is not an escalation: it carries exactly the rights of the user it