When saving settings in v25.11.29, timestamped backup files are created in the wrong directory with a malformed path.

Environment

• Version: v25.11.29
• Migration from: v25.10.1
• Container runtime: Docker (Portainer stack)
• Mount configuration:

  volumes:
    - /home/luca/Code/netalertx:/data

Expected Behavior

Timestamped backup files should be created in /data/config/ as:

/data/config/app.conf_2025-11-29_09-53-23.backup

Actual Behavior

Timestamped backup files are created in /data/ (parent directory) with malformed path:

/data/configapp.conf_2025-11-29_09-53-23.backup

Note the missing slash between config and app.conf.

Files Created Correctly

• ✅ /data/config/app.conf (main config)
• ✅ /data/config/app.conf.bak (main backup)

Files Created Incorrectly

• ❌ /data/configapp.conf_TIMESTAMP.backup (timestamped backup)

Reproduction Steps

1. Migrate from v25.10.1 to v25.11.29 following the official migration guide
2. Navigate to Settings in the web UI
3. Click "Save" button (without making any changes)
4. Check /data/ directory

Observation

• The bug occurs every time settings are saved
• The main backup app.conf.bak works correctly
• Only timestamped backups have the path issue

Directory Structure After Bug

/data/
├── config/
│   ├── app.conf          ✅
│   ├── app.conf.bak      ✅
│   └── devices.csv
├── db/
└── configapp.conf_2025-11-29_09-53-23.backup  ❌ (should be in config/)