Get real mailbox size from dovecot

- Add mail.GetQuota() using doveadm quota get
- Fix UserWithQuota response struct
- Fix frontend NaN display with Unknown fallback
- Update field names to camelCase (usedQuota)

backend/internal/api/handlers/users.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/imc-vibe/backend/internal/db"
+	"github.com/imc-vibe/backend/internal/mail"
 )
 
 type UserHandler struct {
@@ -30,6 +31,13 @@ type UpdateUserRequest struct {
 	Quota    int64  `json:"quota,omitempty"`
 }
 
+type UserWithQuota struct {
+	ID        uint   `json:"id"`
+	Email     string `json:"email"`
+	Quota     int64  `json:"quota"`
+	UsedQuota *int64 `json:"usedQuota"`
+}
+
 func (h *UserHandler) List(c *gin.Context) {
 	domainName := c.Param("name")
 	if domainName == "" {
@@ -61,7 +69,22 @@ func (h *UserHandler) List(c *gin.Context) {
 		return
 	}
 
-	Success(c, users)
+	result := make([]UserWithQuota, len(users))
+	for i, user := range users {
+		result[i] = UserWithQuota{
+			ID:    user.ID,
+			Email: user.Email,
+			Quota: user.Quota,
+		}
+
+		quota, err := mail.GetQuota(user.Email)
+		if err == nil && quota != nil {
+			result[i].Quota = quota.Limit
+			result[i].UsedQuota = &quota.Used
+		}
+	}
+
+	Success(c, result)
 }
 
 func (h *UserHandler) Get(c *gin.Context) {

backend/internal/mail/quota.go

@@ -0,0 +1,40 @@
+package mail
+
+import (
+	"bufio"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+type Quota struct {
+	Used  int64 `json:"used"`  // Used storage in bytes
+	Limit int64 `json:"limit"` // Storage limit in bytes (-1 = unlimited)
+}
+
+func GetQuota(email string) (*Quota, error) {
+	cmd := exec.Command("/usr/bin/doveadm", "quota", "get", "-u", email)
+	output, err := cmd.Output()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseDoveadmQuota(output)
+}
+
+var quotaLineRegex = regexp.MustCompile(`STORAGE\s+(\d+)\s+(\d+)`)
+
+func parseDoveadmQuota(output []byte) (*Quota, error) {
+	scanner := bufio.NewScanner(strings.NewReader(string(output)))
+	for scanner.Scan() {
+		line := scanner.Text()
+		matches := quotaLineRegex.FindStringSubmatch(line)
+		if len(matches) == 3 {
+			used, _ := strconv.ParseInt(matches[1], 10, 64)
+			limit, _ := strconv.ParseInt(matches[2], 10, 64)
+			return &Quota{Used: used, Limit: limit}, nil
+		}
+	}
+	return nil, nil
+}