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/auth.go

@@ -345,6 +345,10 @@ func handleMe(db *sql.DB) http.HandlerFunc {
 			serverError(w, r, err)
 			return
 		}
+		if user.Channels, err = loadChannels(r.Context(), db, caller.ID, true); err != nil {
+			serverError(w, r, err)
+			return
+		}
 		var hash sql.NullString
 		var dismissed *int64
 		if err := db.QueryRowContext(r.Context(),