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

cmd/terdut/main.go

@@ -11,6 +11,7 @@ import (
 	"git.ryuvia.com/niklas/terdut-server/internal/api"
 	"git.ryuvia.com/niklas/terdut-server/internal/config"
 	"git.ryuvia.com/niklas/terdut-server/internal/db"
+	"git.ryuvia.com/niklas/terdut-server/internal/notify"
 )
 
 var version = "dev"
@@ -31,12 +32,11 @@ func main() {
 		log.Fatalf("migrate: %v", err)
 	}
 
-	notify := api.NotifyConfig{
-		BaseURL:       cfg.NtfyURL,
-		Token:         cfg.NtfyToken,
-		FallbackTopic: cfg.NtfyFallbackTopic,
-		PublicURL:     cfg.PublicURL,
-		RepeatEvery:   cfg.NotifyRepeat,
+	notifyCfg := api.NotifyConfig{
+		Providers:   notifyProviders(cfg),
+		Fallback:    notifyFallback(cfg),
+		PublicURL:   cfg.PublicURL,
+		RepeatEvery: cfg.NotifyRepeat,
 	}
 
 	// The behaviour knobs move into the database on first start, after which an
@@ -49,7 +49,7 @@ func main() {
 		log.Fatalf("%v", err)
 	}
 
-	router := api.NewRouter(database, notify, cfg, version)
+	router := api.NewRouter(database, notifyCfg, cfg, version)
 
 	srv := &http.Server{
 		Addr:         cfg.Addr,
@@ -62,8 +62,8 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
-	go api.StartArchiver(ctx, database, cfg.ArchiveAfter, cfg.StaleAfter, notify)
-	go api.StartNotifier(ctx, database, notify)
+	go api.StartArchiver(ctx, database, cfg.ArchiveAfter, cfg.StaleAfter, notifyCfg)
+	go api.StartNotifier(ctx, database, notifyCfg)
 
 	go func() {
 		log.Printf("terdut-server %s listening on %s", version, cfg.Addr)
@@ -80,3 +80,23 @@ func main() {
 		log.Printf("shutdown: %v", err)
 	}
 }
+
+// notifyProviders registers the notification providers this install has
+// configuration for. A provider with no configuration is simply absent, which is
+// what keeps it out of the web UI's list of channel types.
+func notifyProviders(cfg config.Config) *notify.Registry {
+	var providers []notify.Provider
+	if cfg.NtfyURL != "" {
+		providers = append(providers, notify.NewNtfy(cfg.NtfyURL, cfg.NtfyToken))
+	}
+	return notify.NewRegistry(providers...)
+}
+
+// notifyFallback is where an incident goes when nobody on call can be reached.
+// Only ntfy has a fallback setting today; a second provider adds its own.
+func notifyFallback(cfg config.Config) notify.Target {
+	if cfg.NtfyURL != "" && cfg.NtfyFallbackTopic != "" {
+		return notify.Target{Type: notify.TypeNtfy, Address: cfg.NtfyFallbackTopic}
+	}
+	return notify.Target{}
+}