feat: send email notifications for missed messages

- New cron job in cluster
- Handle presence and other concerns
- Toggle in app to disable email notifications
- Handles other edge cases such as cooldown period
- Simple html email with simple message

Closes #117

go/internal/human/models.go

@@ -3,8 +3,10 @@ package human
 import "time"
 
 type Human struct {
-	ID          string
-	Email       string
-	EmailPrefix string
-	CreatedAt   time.Time
+	ID                          string
+	Email                       string
+	EmailPrefix                 string
+	EmailNotificationsEnabled   bool
+	LastEmailNotificationSentAt *time.Time
+	CreatedAt                   time.Time
 }