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.

internal/model/user_membership.go

@@ -0,0 +1,29 @@
+package model
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// UserMembership 表示会员订阅记录（DDL 见 docs/sql/users.sql）。
+type UserMembership struct {
+	ID        uint           `gorm:"primarykey" json:"id"`
+	CreatedAt time.Time      `json:"created_at"`
+	UpdatedAt time.Time      `json:"updated_at"`
+	DeletedAt gorm.DeletedAt `gorm:"index" json:"-"`
+
+	MiniProgramID uint `gorm:"index:idx_membership_user,priority:1;index:idx_membership_status,priority:1" json:"mini_program_id"`
+	UserID        uint `gorm:"index:idx_membership_user,priority:2;index:idx_membership_status,priority:2" json:"user_id"`
+
+	Plan   string `gorm:"size:30" json:"plan"`
+	Status string `gorm:"size:20;index:idx_membership_status,priority:3" json:"status"`
+
+	StartsAt time.Time `json:"starts_at"`
+	EndsAt   time.Time `gorm:"index:idx_membership_user,priority:3;index:idx_membership_status,priority:4" json:"ends_at"`
+}
+
+func (UserMembership) TableName() string {
+	return "user_memberships"
+}
+