chore: remove all NestJS backend references, replace with Hono/Bun

- Delete nestjs-backend.md guideline (replaced by hono-server.md)
- Delete Dockerfile.nestjs-base and Dockerfile.nestjs templates
- Delete stale BACKEND_ARCHITECTURE.md doc (NestJS-era, obsolete)
- Update CLAUDE.md, GUIDELINES.md, authentication.md to Hono/Bun first
- Update all app CLAUDE.md files: backend/ → server/, NestJS → Hono+Bun
- Update all app package.json files: @*/backend → @*/server
- Update docs: LOCAL_DEVELOPMENT, PORT_SCHEMA, ENVIRONMENT_VARIABLES,
  DATABASE_MIGRATIONS, MAC_MINI_SERVER, PROJECT_OVERVIEW
- Update scripts: generate-env.mjs, setup-databases.sh, build-app.sh
- Update CI/CD: cd-macmini.yml backend → server paths
- Update Astro docs site: @chat/backend → @chat/server

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/PROJECT_OVERVIEW.md

@@ -412,7 +412,7 @@ nutriphi/
 │   ├── mobile/       # Expo React Native App (@nutriphi/mobile)
 │   ├── web/          # SvelteKit Web App (@nutriphi/web)
 │   └── landing/      # Astro Landing Page (@nutriphi/landing)
-├── backend/          # NestJS API Server (@nutriphi/backend)
+├── server/           # Hono/Bun server (@nutriphi/server)
 ```
 
 #### API Endpoints