fix(providers): rename muse-spark provider identifier to meta

The provider key was named "muse-spark" but models.dev's remote pricing
catalogue keys it as "meta", so the merge lookup in PriceCatalogue never
matched and the "not found in remote pricing" warning fired on every
refresh. Renamed the identifier (constants, MuseSparkProvider class,
CLI mode, HTTP route, dashboard UI, bundled pricing key) to "meta"
throughout, with a paired SQLite/Postgres migration to update any
existing accounts.provider = 'muse-spark' rows. Model ID strings like
muse-spark-1.2 are unchanged, matching models.dev's own naming.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>