Create a unified "Entity" table for more efficient searching.

[josiah-wolf-oberholtzer]

Merge SQLArtist and SQLLabel into a single table. This can be extended to include Master releases.

• id, primary key
• entity_id
• entity_type (1=artist, 2=label, 3=master)
• name
• description (?)

[josiah-wolf-oberholtzer]

Done.