[Feature]: add VNDB metadata provider

I would like to contribute VNDB as a metadata provider for visual novels.

The implementation includes:

• VNDB Kana API integration
• Manual VNDB ID assignment
• Metadata, aliases, tags, developers, release dates, covers and screenshots
• Configurable metadata and artwork priority
• Scan scope controls to respect VNDB rate limits
• Tests and PostgreSQL migration coverage

The feature is intentionally independent from custom folder layouts and platform fallback rules. Would maintainers be open to reviewing a PR for this?

AI assistance disclosure: this proposal and the implementation were developed with substantial assistance from Codex. The code, tests, migration, and documentation will be reviewed and verified before submission.