Enhance AI and Redis integration for smoke logging features

- Added AI configuration options to .env.example and config.go for OpenAI integration.
- Implemented Redis caching for session management in main.go and auth middleware.
- Updated smoke logging service to support real smoking time (`smoke_at`) and AI advice retrieval.
- Enhanced API routes to include endpoints for AI advice and unlock functionality for non-members.
- Improved database schema with new tables for AI advice and unlock records.
- Expanded documentation to cover new AI features and Redis caching implementation.

docs/membership/API.md

@@ -0,0 +1,71 @@
+# 会员兑换码 API
+
+所有接口前缀：`/api/v1`  
+除登录外都需要：`Authorization: Bearer <session_key>`（见：`docs/common/auth.md`）
+
+## 1) 生成兑换码（后台口令）
+
+`POST /api/v1/membership/redeem_codes`
+
+Header：
+- `X-Admin-Token: <ADMIN_API_TOKEN>`
+
+请求体：
+```json
+{
+  "count": 10,
+  "plan": "month",
+  "duration_days": 30,
+  "expires_at": "2026-12-31 23:59:59",
+  "max_uses": 1
+}
+```
+
+说明：
+- 需要在环境变量配置 `ADMIN_API_TOKEN`，否则接口会返回 503。
+- 兑换码明文只在生成时返回；服务端只保存 `sha256(code)`。
+
+成功响应（示例）：
+```json
+{
+  "code": 200,
+  "message": "success",
+  "data": {
+    "count": 10,
+    "codes": [
+      { "code": "ABCD...1234", "plan": "month" }
+    ]
+  }
+}
+```
+
+## 2) 使用兑换码开通/延长会员
+
+`POST /api/v1/membership/redeem`
+
+请求体：
+```json
+{
+  "code": "ABCD...1234"
+}
+```
+
+成功响应（示例）：
+```json
+{
+  "code": 200,
+  "message": "success",
+  "data": {
+    "plan": "month",
+    "starts_at": "2026-01-03T10:00:00+08:00",
+    "ends_at": "2026-02-02T10:00:00+08:00",
+    "extended": false,
+    "code_suffix": "X7K9Q2"
+  }
+}
+```
+
+常见错误：
+- `400`：兑换码无效/过期/已被使用/不可用
+- `401`：未登录或 token 失效
+