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>

scripts/mac-mini/build-app.sh

@@ -111,11 +111,6 @@ build_base_images() {
   $DOCKER build -f "$PROJECT_ROOT/docker/Dockerfile.sveltekit-base" -t sveltekit-base:local "$PROJECT_ROOT"
   echo "sveltekit-base:local built."
   echo ""
-
-  echo "=== Building nestjs-base image ==="
-  $DOCKER build -f "$PROJECT_ROOT/docker/Dockerfile.nestjs-base" -t nestjs-base:local "$PROJECT_ROOT"
-  echo "nestjs-base:local built."
-  echo ""
 }
 
 build_services() {