Batch import/export

[boldandbrad]

Describe the problem
Users lose their listening history when switching to/from last.fm, making it difficult to migrate or backup scrobble data.

Describe the solution you'd like
Add spin import and spin export commands that:

• Read/write scrobbles to/from JSON/CSV files in last.fm's export format
• Validates and deduplicates against existing scrobbles
• Submits in batches to avoid rate limiting
• Shows progress and summary of imported/exported tracks

Example:

spin import path/to/export.json
"Imported 1,234 scrobbles (skipped 56 duplicates)"

spin export path/to/export.json

Describe alternatives you've considered
Manual re-scrobbling is impractical for thousands of tracks.

Additional context/considerations

• Support last.fm's standard export format
• May need to respect last.fm's submission limits
• Consider rate limiting between submissions
• Potential to handle libre.fm import/export as well: Libre.fm support #3