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/service_accounts_test.go

@@ -141,7 +141,7 @@ func TestServiceAccount_TeamScopeIsBoundToItsOwnTeam(t *testing.T) {
 	teamB := createTeamAs(t, s, instanceKey, "team-b")
 	keyA := createServiceAccount(t, s, instanceKey, "team-a-sa", models.ServiceAccountScopeTeam, teamA)
 
-	policy := map[string]any{"repeat_count": 0, "fallback_topic": "", "levels": []any{}}
+	policy := map[string]any{"repeat_count": 0, "fallback": map[string]string{"type": "", "address": ""}, "levels": []any{}}
 
 	resp := s.reqAs(t, keyA, http.MethodPut, "/api/teams/"+id64(teamA)+"/escalation", policy)
 	if resp.StatusCode != http.StatusOK {