feat: switch foundry backend from Bun to Node.js

- Replace Bun.serve() with @hono/node-server (already a dependency)
- Replace Bun-specific debug APIs (Bun.gc, Bun.heapStats,
  Bun.generateHeapSnapshot) with Node.js equivalents (globalThis.gc)
- Update all three Dockerfiles from oven/bun base images to
  node:22-bookworm-slim
- Remove @types/bun devDependency
- Update package.json scripts to use node/npx instead of bun

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

foundry/packages/backend/package.json

@@ -7,10 +7,10 @@
   "types": "dist/index.d.ts",
   "scripts": {
     "build": "tsup src/index.ts --format esm",
-    "db:generate": "find src/actors -name drizzle.config.ts -exec pnpm exec drizzle-kit generate --config {} \\; && \"$HOME/.bun/bin/bun\" src/actors/_scripts/generate-actor-migrations.ts",
+    "db:generate": "find src/actors -name drizzle.config.ts -exec pnpm exec drizzle-kit generate --config {} \\; && npx tsx src/actors/_scripts/generate-actor-migrations.ts",
     "typecheck": "tsc --noEmit",
-    "test": "$HOME/.bun/bin/bun x vitest run",
-    "start": "bun dist/index.js start"
+    "test": "npx vitest run",
+    "start": "node dist/index.js start"
   },
   "dependencies": {
     "@e2b/code-interpreter": "^2.3.3",
@@ -31,7 +31,6 @@
     "zod": "^4.1.5"
   },
   "devDependencies": {
-    "@types/bun": "^1.3.9",
     "tsup": "^8.5.0"
   }
 }