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.

package.json

@@ -7,7 +7,7 @@
     "node": ">=22"
   },
   "scripts": {
-    "build": "tsc --project tsconfig.json",
+    "build": "tsc --project tsconfig.json && node scripts/copy-assets.mjs",
     "dev": "tsx watch src/main.ts",
     "start": "node dist/main.js",
     "test": "vitest run",