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.

tsconfig.build.json

@@ -0,0 +1,14 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "noEmit": false,
+    "declaration": true,
+    "declarationMap": false,
+    "sourceMap": false,
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "allowImportingTsExtensions": false
+  },
+  "include": ["src/**/*"],
+  "exclude": ["tests/**/*", "node_modules", "dist"]
+}