Fix Seafile MySQL dump authentication by using root user

Update the Seafile MySQL dump hook to use the root user by default
instead of seafile user, which resolves authentication issues. Add
SEAFILE_MYSQL_USER environment variable for user customization.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

hooks/seafile-docker-mysql-backup.sh

@@ -22,11 +22,15 @@ else
   echo "⚠️  /etc/borgmatic/.env non trouvé"
 fi
 
+# Utilisateur MySQL (par défaut root, configurable via .env)
+MYSQL_USER="${SEAFILE_MYSQL_USER:-root}"
+
 # Vérifier que le mot de passe est défini
 if [ -z "$SEAFILE_DB_PASSWORD" ]; then
   echo "❌ SEAFILE_DB_PASSWORD n'est pas défini dans /etc/borgmatic/.env"
   exit 1
 fi
+echo "✓ Utilisateur MySQL: $MYSQL_USER"
 echo "✓ Mot de passe MySQL défini"
 
 # Vérifier que le conteneur existe et tourne
@@ -41,7 +45,7 @@ echo "✓ Conteneur $MYSQL_CONTAINER trouvé"
 # Dump unique avec toutes les bases
 echo "→ Dump des bases Seafile..."
 if timeout 300 docker exec "$MYSQL_CONTAINER" mysqldump \
-  -u seafile -p"${SEAFILE_DB_PASSWORD}" \
+  -u "$MYSQL_USER" -p"${SEAFILE_DB_PASSWORD}" \
   --single-transaction --quick --databases \
   ccnet_db seafile_db seahub_db > "$BACKUP_DIR/seafile-all-$DATE.sql" 2>&1; then