feat(downloads): auto-restore downloaded tracks on reinstall + audio permission prompt

[X-MAN-TG]

When users reinstall the app or clear data, previously downloaded audio files remain in storage (e.g. Music/LastWave), but the local database entries are lost.

This adds automatic background restoration so users don't lose their offline library:

• Auto-restore scan: On first launch, scans the download directory for existing audio tracks, reads embedded metadata/tags, and restores them directly into the Room database and downloads UI without re-downloading.
• Runtime audio permission prompt: Requests READ_MEDIA_AUDIO (API 33+) or READ_EXTERNAL_STORAGE (API <= 32) on first launch chained after notification permission if needed.
• Live background sync: As soon as permission is granted, runs the scan in the background so tracks appear immediately without restarting the app.
• Graceful fallback: If denied, it leaves the scan flag unset and doesn't re-prompt in the same session. Manual re-scan is also supported in Downloads settings.
• Includes unit tests for permission mapping and scan deduplication.