fix: clean up orphaned .sql on compression failure and fix stale startup log

- backup-lib: delete uncompressed .sql file in catch block when gzip
  compression fails, preventing silent disk usage accumulation
- server: replace stale retentionDays scalar with retentionSource in
  startup log since retention is now read from DB on each backup tick

server/src/index.ts

@@ -670,7 +670,7 @@ export async function startServer(): Promise<StartedServer> {
     logger.info(
       {
         intervalMinutes: config.databaseBackupIntervalMinutes,
-        retentionDays: config.databaseBackupRetentionDays,
+        retentionSource: "instance-settings-db",
         backupDir: config.databaseBackupDir,
       },
       "Automatic database backups enabled",