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/deadman_test.go

@@ -29,12 +29,12 @@ func deadmanTS(t *testing.T, cfg api.DeadmanConfig) (*ts, *fakeNtfy) {
 	t.Helper()
 	f := newFakeNtfy(t)
 	s := newDeadmanTS(t, cfg, api.NotifyConfig{
-		BaseURL:   f.URL,
+		Providers: f.providers(),
 		PublicURL: "https://terdut.example.com",
 	})
 
 	putOnCall(t, s, 1)
-	setTopic(t, s, 1, "terdut-admin")
+	addNtfyChannel(t, s, 1, "terdut-admin")
 	return s, f
 }