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/settings.go

@@ -11,6 +11,7 @@ import (
 
 	"git.ryuvia.com/niklas/terdut-server/internal/config"
 	"git.ryuvia.com/niklas/terdut-server/internal/models"
+	"git.ryuvia.com/niklas/terdut-server/internal/notify"
 	"github.com/go-chi/chi/v5"
 )
 
@@ -111,7 +112,7 @@ type settingValue struct {
 	MaxSeconds  int64  `json:"max_seconds"`
 }
 
-func handleGetSettings(db *sql.DB, cfg config.Config) http.HandlerFunc {
+func handleGetSettings(db *sql.DB, cfg config.Config, providers *notify.Registry) http.HandlerFunc {
 	settings := NewSettings(db)
 	return func(w http.ResponseWriter, r *http.Request) {
 		out := settingsResponse{
@@ -128,11 +129,12 @@ func handleGetSettings(db *sql.DB, cfg config.Config) http.HandlerFunc {
 				// Never the ntfy token or the DSN: both are credentials, and an
 				// admin page that renders them turns a browser tab into a place
 				// they leak from.
-				"ntfy_url":        cfg.NtfyURL,
-				"ntfy_configured": strconv.FormatBool(cfg.NtfyURL != ""),
-				"ntfy_token_set":  strconv.FormatBool(cfg.NtfyToken != ""),
-				"public_url":      cfg.PublicURL,
-				"listen_address":  cfg.Addr,
+				"notification_providers": strings.Join(providerTypes(providers), ", "),
+				"ntfy_url":               cfg.NtfyURL,
+				"ntfy_configured":        strconv.FormatBool(cfg.NtfyURL != ""),
+				"ntfy_token_set":         strconv.FormatBool(cfg.NtfyToken != ""),
+				"public_url":             cfg.PublicURL,
+				"listen_address":         cfg.Addr,
 			},
 		}
 		for key, b := range settingBounds {