feat: initialize project skeleton with Go backend, database, and config

- Go module with standard library + gorilla/sessions + mysql driver
- Config: env-based configuration with .env file support
- Database: MySQL connection with auto-migration for 6 tables
- Seed data: 12 system tags with options, 8 system snippets
- Auth: session cookie middleware
- Handlers: auth, tags, snippets, builder, claude, dashboard, suggestions, settings
- LLM: OpenAI-compatible client with 30s timeout
- Docker: Dockerfile + docker-compose.yml
- .env.example with all configuration options

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

.env.example

@@ -0,0 +1,18 @@
+# 服务配置
+SERVER_PORT=8080
+SESSION_SECRET=your-random-secret-key
+
+# 数据库（连接已有的远程 MySQL 实例）
+DB_HOST=your-mysql-public-ip
+DB_PORT=3306
+DB_USER=your-db-user
+DB_PASSWORD=your-db-password
+DB_NAME=prompt_generator
+
+# 认证
+AUTH_PASSWORD=your-access-password
+
+# LLM
+LLM_API_BASE_URL=https://api.deepseek.com/v1
+LLM_API_KEY=sk-xxx
+LLM_MODEL_NAME=deepseek-chat