Enhance smoking tracking features with AI next smoke time suggestions

- Added new API endpoint `GET /api/v1/smoke/next_smoke_time` to provide AI-generated suggestions for the next smoking time based on user data.
- Introduced a new database table `fa_smoke_ai_next_smoke` to store structured AI time node suggestions.
- Updated smoke handler and service to integrate the new AI next smoke time functionality.
- Enhanced documentation to reflect the new API endpoint and its usage, including details on how to generate AI time nodes.

docs/smoke/README.md

@@ -41,6 +41,7 @@
 涉及表（DDL 见：`docs/sql/smoke.sql`）：
 - `fa_smoke_ai_advice`：按 `uid + advice_date + prompt_version` 缓存建议结果，避免重复调用 AI。
 - `fa_smoke_ai_advice_unlocks`：非会员用户的“每日解锁”记录（按 `uid + unlock_date` 唯一）。
+- `fa_smoke_ai_next_smoke`：保存“AI 下次抽烟时间节点”（每个时间点一条记录），AI 元信息复用 `fa_smoke_ai_advice`（`type=next_smoke_time`）。
 
 建议的权限判断顺序：
 1) 若用户是会员：直接允许生成/获取建议。