fix: externalize better-sqlite3 for Next.js standalone builds

Move better-sqlite3 to optionalDependencies so npm install doesn't
fail on platforms without native build tools. Add it to
serverExternalPackages so Next.js doesn't try to bundle the native
addon into webpack chunks.

Fixes #243

next.config.mjs

@@ -1,6 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   output: "standalone",
+  serverExternalPackages: ["better-sqlite3"],
   images: {
     unoptimized: true
   },

package.json

@@ -15,7 +15,6 @@
     "@monaco-editor/react": "^4.7.0",
     "@xyflow/react": "^12.10.1",
     "bcryptjs": "^3.0.3",
-    "better-sqlite3": "^12.6.2",
     "confbox": "^0.2.4",
     "express": "^5.2.1",
     "fs": "^0.0.1-security",
@@ -38,6 +37,9 @@
     "uuid": "^13.0.0",
     "zustand": "^5.0.10"
   },
+  "optionalDependencies": {
+    "better-sqlite3": "^12.6.2"
+  },
   "devDependencies": {
     "@tailwindcss/postcss": "^4.1.18",
     "esbuild": "^0.27.4",