fix(db): require better-sqlite3 ^12.4.1 so Node 24 gets the native backend (#203) (#216)

better-sqlite3 ^11.0.0 (latest 11.10.0) ships no prebuilt binary for
Node 24's ABI (node-v137) and predates Node 24, so every Node 24 install
silently fell back to the 5-10x-slower WASM backend. Bump to ^12.4.1 —
the first 12.x with the Node 24 prebuild — and raise the engines floor to
Node 20 (Node 18 is EOL and dropped from better-sqlite3 12.x prebuilds).

Verified on macOS Node 24.15.0 (ABI 137): prebuilt binary used with no
compiler (installs even with CC/CXX sabotaged), `codegraph init -i` shows
no WASM banner, and `codegraph status` reports Backend: native. 639/639
tests pass on Node 22.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>