[refactor](alarm): make a readable message structural on every error

An error status with an empty body is a client-breaking bug that review does not reliably catch. Route every JSON and plain-text error through internal/apierr, which substitutes the code canonical message when a caller supplies none, and keep the frozen codes in one package shared by the action and WebSocket layers.

src/actions/errors.go

@@ -31,7 +31,6 @@ import (
 	"time"
 
 	"git.dcntrlized.org/Dcntrlized/Alarm/db"
-	"git.dcntrlized.org/Dcntrlized/Alarm/internal/alertstate"
 	"git.dcntrlized.org/Dcntrlized/Alarm/internal/apierr"
 	"git.dcntrlized.org/dcntrlized/dcntrlized-auth/authclient"
 	"github.com/google/uuid"
@@ -40,15 +39,16 @@ import (
 // Frozen v1 error codes, aliased from internal/apierr so the WebSocket layer
 // (which cannot import this package) uses the same strings.
 const (
-	CodeAlertNotFound      = apierr.AlertNotFound
-	CodeAlertTokenInvalid  = apierr.AlertTokenInvalid
-	CodeAlertNotAdmin      = apierr.AlertNotAdmin
-	CodeAlertAlreadyJoined = apierr.AlertAlreadyJoined
-	CodeAlertNotSubscribed = apierr.AlertNotSubscribed
-	CodeAlertState         = apierr.AlertState
-	CodeValidation         = apierr.Validation
-	CodeUnauthorized       = apierr.Unauthorized
-	CodeInternal           = apierr.Internal
+	CodeAlertNotFound         = apierr.AlertNotFound
+	CodeAlertTokenInvalid     = apierr.AlertTokenInvalid
+	CodeAlertNotAdmin         = apierr.AlertNotAdmin
+	CodeAlertAlreadyJoined    = apierr.AlertAlreadyJoined
+	CodeAlertNotSubscribed    = apierr.AlertNotSubscribed
+	CodeAlertState            = apierr.AlertState
+	CodeAlarmTriggeredOffline = apierr.AlarmTriggeredOffline
+	CodeValidation            = apierr.Validation
+	CodeUnauthorized          = apierr.Unauthorized
+	CodeInternal              = apierr.Internal
 )
 
 // maxMessageBytes / maxNameChars bound the free-text fields.
@@ -60,22 +60,11 @@ const (
 	maxPlatformLen  = 32
 )
 
-// errorEnvelope is the platform error body.
-type errorEnvelope struct {
-	Error struct {
-		Code    string `json:"code"`
-		Message string `json:"message"`
-	} `json:"error"`
-}
-
-// writeError emits {"error":{"code","message"}}.
+// writeError emits {"error":{"code","message"}}. It delegates to apierr.Write,
+// which guarantees both fields are non-empty: this package can pass a literal
+// message, but the invariant is enforced centrally (see internal/apierr).
 func writeError(w http.ResponseWriter, status int, code, message string) {
-	var body errorEnvelope
-	body.Error.Code = code
-	body.Error.Message = message
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-	_ = json.NewEncoder(w).Encode(body)
+	apierr.Write(w, status, code, message)
 }
 
 // writeJSON emits a success body.
@@ -180,13 +169,6 @@ func validateMessage(raw string) (string, error) {
 	return raw, nil
 }
 
-func validateAutoStop(seconds int) error {
-	if seconds < 0 || seconds > alertstate.MaxAutoStopSeconds {
-		return fmt.Errorf("autoStopSeconds must be between 0 and %d", alertstate.MaxAutoStopSeconds)
-	}
-	return nil
-}
-
 func validateDeviceID(raw string) (string, error) {
 	id := strings.TrimSpace(raw)
 	if id == "" {