feat: initialize AutoTeacher Next.js project structure

.env.example

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+# Database
+DATABASE_URL="postgresql://postgres:postgres@localhost:5432/autoteacher_next"
+
+# S3-compatible storage
+AUTOTEACHER_S3_BUCKET="autoteacher"
+AUTOTEACHER_S3_REGION="us-east-1"
+# AUTOTEACHER_S3_ENDPOINT_URL="https://s3.example.com"
+AUTOTEACHER_S3_ACCESS_KEY_ID="changeme"
+AUTOTEACHER_S3_SECRET_ACCESS_KEY="changeme"
+# Optional public base URL for signed/accelerated delivery
+# AUTOTEACHER_S3_PUBLIC_BASE_URL="https://cdn.example.com"
+AUTOTEACHER_S3_USE_SSL="true"
+
+# Clerk Authentication (fill with your project's keys)
+NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY=""
+CLERK_SECRET_KEY=""
+
+# Grading service
+# API 密钥需要在 .env.local 中填写真实值
+GRADING_API_KEY=""
+# 如需自定义地址，可以在 .env.local 中设置该变量
+# GRADING_API_URL="https://www.chataiapi.com/v1beta/models/gemini-2.5-pro:generateContent"