Adds migration 0003_seed_seasons.sql that creates the initial
season "Season 1: The Founding" with status=active. The index
builder already has code to generate seasons/index.json from the
database (generateSeasonsIndex in generator.go, fetchSeasons in db.go).
The season starts 7 days ago and has no end date or champion yet,
allowing it to serve as the active season for the platform.
Closes: bf-4w0x
