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/membership/model/redeem_code.go

@@ -0,0 +1,56 @@
+package model
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// MembershipRedeemCode 存储会员兑换码（只存 hash，不存明文 code）。
+type MembershipRedeemCode 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:"-"`
+
+	// CodeHash 是 code 的 sha256(hex)；避免 DB 泄漏导致所有兑换码被直接使用。
+	CodeHash string `gorm:"size:64;uniqueIndex" json:"-"`
+	// CodeSuffix 用于展示/审计（例如最后 6 位）
+	CodeSuffix string `gorm:"size:16;index" json:"code_suffix"`
+
+	Plan         string     `gorm:"size:30" json:"plan"`
+	DurationDays int        `json:"duration_days"`
+	ExpiresAt    *time.Time `json:"expires_at,omitempty"`
+
+	MaxUses  int `gorm:"default:1" json:"max_uses"`
+	UsedUses int `gorm:"default:0" json:"used_uses"`
+
+	Status string `gorm:"size:20;default:active" json:"status"` // active/disabled
+}
+
+func (MembershipRedeemCode) TableName() string {
+	return "membership_redeem_codes"
+}
+
+type MembershipRedemption 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_redeem_user_date,priority:1" json:"mini_program_id"`
+	UserID        uint `gorm:"index:idx_redeem_user_date,priority:2" json:"user_id"`
+
+	RedeemCodeID uint   `gorm:"index" json:"redeem_code_id"`
+	CodeSuffix   string `gorm:"size:16" json:"code_suffix"`
+
+	MembershipID uint `gorm:"index" json:"membership_id"`
+
+	ClientIP  string `gorm:"size:64" json:"client_ip,omitempty"`
+	UserAgent string `gorm:"size:255" json:"user_agent,omitempty"`
+}
+
+func (MembershipRedemption) TableName() string {
+	return "membership_redemptions"
+}
+