fix(fifo): ContentBasedDeduplication + dedup window survives delete (AWS semantics)

- ContentBasedDeduplication=true: dedupId computed from body SHA-256 when not provided
- do NOT drop dedupId on message delete: AWS keeps the 5-minute window regardless
  (previously re-send after delete delivered the message again)
- utils: GetSHA256Hash

app/utils/utils.go

@@ -2,6 +2,7 @@ package utils
 
 import (
 	"crypto/md5"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/binary"
 	"encoding/hex"
@@ -97,6 +98,13 @@ func GetMD5Hash(text string) string {
 	return hex.EncodeToString(hasher.Sum(nil))
 }
 
+// GetSHA256Hash — SHA-256 (hex) от значения. Используется для
+// ContentBasedDeduplication: AWS сам вычисляет MessageDeduplicationId из тела.
+func GetSHA256Hash(text string) string {
+	sum := sha256.Sum256([]byte(text))
+	return hex.EncodeToString(sum[:])
+}
+
 func HashAttributes(attributes map[string]models.MessageAttribute) string {
 	hasher := md5.New()