chore: add package.json, tsconfig pair, and vitest config

ESM-only, Node 20+, ships pre-compiled dist/. Peer dependency on
better-sqlite3 >=11 <13. Dev pins: typescript ^5.5, vitest ^4.0
(resolved to 4.1.6), better-sqlite3 12.6.2 (matches authd).

tsconfig.json (noEmit) is the dev/test config; tsconfig.build.json
extends it and emits .js + .d.ts to dist/. Relative imports inside src/
use .js extensions so the emitted JS resolves correctly at runtime.

vitest.config.ts

@@ -0,0 +1,5 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: { globals: false, environment: 'node' },
+});