feat: complete Phase 3 + new production roadmap for Web/ERP

- Thread-safety: locks in LSMTree and Graph engines
- Raft network transport: async TCP, serialization, heartbeat, 3-node election test
- CI/CD: GitHub Actions workflow
- Cleanup: remove dead code, unused imports, build artifacts
- New PLAN.md targeting production Web/ERP readiness
- 216 tests passing

src/barabadb/core/gossip.nim

@@ -1,6 +1,5 @@
 ## Gossip Protocol — membership and failure detection
 import std/tables
-import std/sets
 import std/random
 import std/monotimes