feat: homework multi-image upload and crop

.env.example

@@ -0,0 +1,14 @@
+DOMAIN=homework.hao.work
+
+LLM_BASE_URL=https://one.hao.work/v1/chat/completions
+LLM_API_KEY=your_api_key_here
+LLM_MODEL=qwen3-max
+LLM_TIMEOUT_SECONDS=90
+LLM_MAX_RETRIES=3
+UPLOAD_DIR=/data/uploads
+
+ADMIN_USER=admin
+ADMIN_PASS=whoami139
+
+BACKEND_PORT=8080
+NEXT_PUBLIC_API_BASE=/api