PR #881 (commit 755e424) fixed the race condition between normal consolidation
and /new consolidation, but did so by making /new wait for consolidation to
complete before returning. This hurts user experience - /new should be instant.
This PR restores the original immediate-return behavior while keeping safety:
1. **Immediate return**: Session clears and user sees "New session started" right away
2. **Background archival**: Consolidation runs in background via asyncio.create_task
3. **Serialized consolidation**: Uses the same lock as normal consolidation via
`memory_consolidator.get_lock()` to prevent concurrent writes
If consolidation fails after session clear, archived messages may be lost.
This is acceptable because:
- User already sees the new session and can continue working
- Failure is logged for debugging
- The alternative (blocking /new on every call) hurts UX for all users
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