Add comma key to mark artist as caught up on releases

[Jason-Abbott]

• Add reviewed_at column to schema.sql and migration v19
• Add SetReviewedAt mutation and update AlbumArtists query
• Regenerate sqlc
• Add comma key handler to TUI
• Update renderItem badge logic for reviewed_at
• Update renderPane with NEWER RELEASES / REVIEWED split
• Update help text
• Update test version checks
• Build, test, lint

Summary of Changes

Added reviewed_at column to artists table (migration v19). Pressing , toggles the reviewed date: sets to today if empty, clears if set. The * badge only shows for releases newer than reviewed_at. The discography pane splits non-local releases into NEWER RELEASES (unreviewed) and REVIEWED (acknowledged) sections.

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