[FEATURE] Add manual compaction API (level/key-range)

[feichai0017]

🚀 Is your feature request related to a problem?

Compaction is currently background-driven only. Operators and experiments lack explicit controls for forcing compaction at specific levels/ranges.

💡 Proposed Solution

Add a manual compaction API and CLI entrypoint.

Examples:

• compact a full level,
• compact a key range,
• trigger with optional wait/status output.

🎨 Alternatives Considered

1. Keep only automatic compaction scheduler.
2. Use offline tools/scripts touching internal files.

Manual API is safer and more operable.

📝 Technical Details / Implementation Plan

• Define internal compaction job request (level/range/options).
• Expose API in DB and command entry in cmd/nokv.
• Add tests for correctness, idempotence, and interaction with running background compactions.

🧐 Additional Context

Manual compaction is useful for benchmarking, incident mitigation, and maintenance windows.

[ByteByteUp]

Is anyone currently working on this issue? If not, I‘d be happy to take it on.