feat: self-service user signup (#672)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/internal/model/signup_token.go

@@ -0,0 +1,28 @@
+package model
+
+import (
+	"time"
+
+	datatype "github.com/pocket-id/pocket-id/backend/internal/model/types"
+)
+
+type SignupToken struct {
+	Base
+
+	Token      string            `json:"token"`
+	ExpiresAt  datatype.DateTime `json:"expiresAt" sortable:"true"`
+	UsageLimit int               `json:"usageLimit" sortable:"true"`
+	UsageCount int               `json:"usageCount" sortable:"true"`
+}
+
+func (st *SignupToken) IsExpired() bool {
+	return time.Time(st.ExpiresAt).Before(time.Now())
+}
+
+func (st *SignupToken) IsUsageLimitReached() bool {
+	return st.UsageCount >= st.UsageLimit
+}
+
+func (st *SignupToken) IsValid() bool {
+	return !st.IsExpired() && !st.IsUsageLimitReached()
+}