fix(images): restore custom placeholder for coverless items

[ArtwoXYZ]

What does this PR do?

Restores the configured default image for coverless items in custom plugins.

Root cause

The custom plugin formatter historically turns an empty cover_image into the generic DVinyl logo. Since the image gallery change, the shared formatter reads that value back as if it were a real item image, so it wins over the custom plugin placeholder after saving. The add-item preview is not affected because it reads the plugin placeholder directly.

Fix

Ignore the generic and plugin-specific placeholders when building the list of actual item images. Real covers keep their priority, while a coverless item falls back to the plugin default at render time as intended. No data migration is needed.

Related issue

Fixes #154

Type of change

• 🐛 Bug fix
• ✨ New feature
• 🧩 New or updated plugin
• 📖 Documentation
• 🌍 Translation
• 🧹 Refactor or chore

Checklist

• make typecheck passes
• I tested my changes locally
• I updated the docs or translations if needed
• My change is focused on a single thing

Screenshots

Not applicable, there is no UI layout change.

Anything else?

Validation performed:

• npx tsc --noEmit
• Targeted runtime check for both a coverless custom item and an item with a real cover
• Docker build and application smoke test with an isolated MongoDB