Simplify musup to single-argument CLI

[Jason-Abbott]

Goal

Change musup from a multi-command CLI (scan, check, version) to a simple single-argument CLI:

musup <years>

Example: musup 1 shows all artists with an album release in the last 1 year that is newer than what the user has in local files.

Motivation

This substantially simplifies:

• What data needs to be stored (just artists + their latest local release date)
• What queries are needed for MusicBrainz (single query per artist: "any album in last N years?")
• The overall UX (one command, one argument)

Tasks

• Design the new simplified data model (what columns/tables are needed)
• Remove Cobra subcommands (scan, check, version) in favor of a single root command
• Accept a single positional argument: number of years (integer)
• Combine scan + check into a single flow: scan local files, then check MusicBrainz for newer releases
• Simplify DB schema to only store what's needed
• Simplify MusicBrainz queries to only fetch what's needed
• Update README and help text
• Update tests

Summary of Changes

Simplified musup from an interactive TUI app to a two-command CLI:

• musup scan [path] — scans music files into the database
• musup [years] — shows artists with new album releases in the last N years (default: 1) that aren't in the local library

What changed:

• DB schema v13: Dropped tracks table, removed followed/reviewed_at/monitor columns from artists
• New NewerReleases query: Single SQL query finds MB albums released since cutoff that aren't in local files
• New internal/check package: Orchestrates MusicBrainz sync (search artist → fetch release groups → upsert albums)
• Simplified MB client: Removed BrowseReleases and all Release/Track/Recording types (no longer fetching individual tracks)
• Deleted internal/tui/: Removed all 11 Bubble Tea TUI files
• Deleted track/prune queries: Removed track.sql, prune.sql and generated code
• Removed TUI dependencies: bubbletea, bubbles, lipgloss dropped from go.mod

Files deleted (16):

• internal/tui/* (11 files)
• internal/db/track.sql, track.sql.go, prune.sql, prune.sql.go

Files created (3):

• internal/check/check.go
• cmd/scan.go

All tests pass, lint clean.

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