schema: add metadata descriptor for 'demo' (as in demo version, not demoscene)

Te represent https://simple.wikipedia.org/wiki/Demo_(software), not the existing https://en.wikipedia.org/wiki/Demoscene

Start by examining the existing metadata descriptor for “demoscene” and the schema entries that define descriptors. Add a separate descriptor for “demo” meaning a software demo, keeping it distinct from demoscene, then confirm the new descriptor is represented correctly in the database.

索引モデルが issue の本文から書いたものです。