feat(backups): gzip compress backups and add retention config to Instance Settings

Compress database backups with gzip (.sql.gz), reducing file size ~83%.
Add backup retention configuration to Instance Settings UI with preset
options (7 days, 2 weeks, 1 month). The backup scheduler now reads
retention from the database on each tick so changes take effect without
restart. Default retention changed from 30 to 7 days.

server/src/config.ts

@@ -216,7 +216,7 @@ export function loadConfig(): Config {
     1,
     Number(process.env.PAPERCLIP_DB_BACKUP_RETENTION_DAYS) ||
       fileDatabaseBackup?.retentionDays ||
-      30,
+      7,
   );
   const databaseBackupDir = resolveHomeAwarePath(
     process.env.PAPERCLIP_DB_BACKUP_DIR ??