feat: add invite generation and validation

internal/collaboration/invite.go

···
+package collaboration
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"time"
+
+	"github.com/limeleaf/diffdown/internal/db"
+	"github.com/limeleaf/diffdown/internal/model"
+)
+
+func GenerateInviteToken() (string, error) {
+	bytes := make([]byte, 32)
+	if _, err := rand.Read(bytes); err != nil {
+		return "", err
+	}
+	hash := sha256.Sum256(bytes)
+	return hex.EncodeToString(hash[:]), nil
+}
+
+func CreateInvite(database *db.DB, documentRKey, createdByDID string) (*model.Invite, error) {
+	token, err := GenerateInviteToken()
+	if err != nil {
+		return nil, err
+	}
+
+	invite := &model.Invite{
+		ID:           db.NewID(),
+		DocumentRKey: documentRKey,
+		Token:        token,
+		CreatedBy:    createdByDID,
+		CreatedAt:    time.Now(),
+		ExpiresAt:    time.Now().Add(7 * 24 * time.Hour),
+	}
+
+	err = database.CreateInvite(invite)
+	return invite, err
+}
+
+func ValidateInvite(database *db.DB, token, documentRKey string) (*model.Invite, error) {
+	invite, err := database.GetInviteByToken(token)
+	if err != nil {
+		return nil, err
+	}
+	if invite.DocumentRKey != documentRKey {
+		return nil, fmt.Errorf("invite does not match document")
+	}
+	if time.Now().After(invite.ExpiresAt) {
+		return nil, fmt.Errorf("invite expired")
+	}
+	return invite, nil
+}

internal/db/db.go

···
 	return err
 }
 
+// --- Invites ---
+
+func (db *DB) CreateInvite(invite *model.Invite) error {
+	_, err := db.Exec(`
+		INSERT INTO invites (id, document_rkey, token, created_by_did, created_at, expires_at)
+		VALUES (?, ?, ?, ?, ?, ?)`,
+		invite.ID, invite.DocumentRKey, invite.Token, invite.CreatedBy, invite.CreatedAt, invite.ExpiresAt)
+	return err
+}
+
+func (db *DB) GetInviteByToken(token string) (*model.Invite, error) {
+	row := db.QueryRow(`SELECT id, document_rkey, token, created_by_did, created_at, expires_at FROM invites WHERE token = ?`, token)
+	var invite model.Invite
+	err := row.Scan(&invite.ID, &invite.DocumentRKey, &invite.Token, &invite.CreatedBy, &invite.CreatedAt, &invite.ExpiresAt)
+	if err != nil {
+		return nil, err
+	}
+	return &invite, nil
+}