fix: User-Struct JSON-Tags ergänzen (active/source/id etc. waren Großbuchstaben)

Frontend erhielt 'Active'/'Source' statt 'active'/'source' →
Inaktiv-Badge und local-Badge zeigten falsche Werte.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/userstore/userstore.go

@@ -23,13 +23,13 @@ const (
 
 // User represents a user account in the system.
 type User struct {
-	ID          int64
-	Username    string
-	Email       string
-	Role        string
-	Source      string // "local" or "ldap"
-	Active      bool
-	CreatedAt   time.Time
+	ID          int64      `json:"id"`
+	Username    string     `json:"username"`
+	Email       string     `json:"email"`
+	Role        string     `json:"role"`
+	Source      string     `json:"source"` // "local" or "ldap"
+	Active      bool       `json:"active"`
+	CreatedAt   time.Time  `json:"created_at"`
 	TenantID    *int64     `json:"tenant_id,omitempty"`
 	TOTPEnabled bool       `json:"totp_enabled"`
 	TOTPResetAt *time.Time `json:"totp_reset_at,omitempty"`