Copy SQL migration files into dist/ as part of build

tsc only emits .ts -> .js; non-TypeScript assets like SQL migration
files don't make it into dist/ by default. The migration runner reads
*.sql from dist/db/migrations/ at runtime in production (relative to
the compiled migrate.js), so the missing files surface as a fatal
ENOENT on container startup.

Fix: small node script (scripts/copy-assets.mjs) using fs.cpSync,
invoked after tsc in the build script. Cross-platform, no new
dependencies. The script is in the Docker build context but not
copied into the runtime stage, so it doesn't bloat the final image.

Verified: pnpm build now produces dist/db/migrations/0001_positions.sql.

scripts/copy-assets.mjs

@@ -0,0 +1,16 @@
+// Copies non-TypeScript build artifacts into dist/ after tsc.
+// Currently: SQL migration files for the runtime migration runner.
+// tsc only emits .ts → .js; everything else has to be copied explicitly.
+
+import { cpSync, existsSync } from 'node:fs';
+
+const assets = [{ src: 'src/db/migrations', dest: 'dist/db/migrations' }];
+
+for (const { src, dest } of assets) {
+  if (!existsSync(src)) {
+    console.error(`copy-assets: source missing: ${src}`);
+    process.exit(1);
+  }
+  cpSync(src, dest, { recursive: true });
+  console.log(`copy-assets: ${src} -> ${dest}`);
+}