Add settings table and Enter key to open artist search URL

[Jason-Abbott]

• Add settings table to schema.sql and migration v20
• Create settings.sql with GetSetting/SetSetting queries
• Regenerate sqlc
• Add Enter key handler and openSearchURL in TUI
• Load search_url via loadArtists into Model
• Update help text
• Update test version checks
• Build, test, lint

Summary of Changes

Added settings table (key-value) with migration v20. Seeded with search_url = https://www.allmusic.com/search/artists/%s. Press Enter on an artist to open their allmusic search page in the default browser. Checked for orphan code — modalConfirmFetch is still used by the . key inactive status fetch.

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