feat: Phase 3 — HTTP REST API + JWT auth + Prometheus metrics

- New HTTP server (core/httpserver.nim) with async HTTP/1.1
- POST /query — execute SQL, return JSON
- GET /health — readiness/liveness probe
- GET /metrics — Prometheus-format counters
- JWT bearer token auth via Authorization header
- Per-request MVCC transaction context isolation
- Fixed B-Tree index mutable access in executor
- All 216 tests pass

src/barabadb/core/server.nim

@@ -4,7 +4,6 @@ import std/asyncnet
 import std/strutils
 import std/os
 import std/endians
-import std/tables
 import config
 import ../protocol/wire
 import ../query/lexer