Fix: use single volume for Fly.io
Fly.io only supports one volume per machine. Consolidate:
- LiteFS data at /data/litefs (persistent)
- Local DB at /data/local.db (persistent)
- FUSE mount at /litefs (virtual, created by LiteFS)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>