feat(auth): invite links for several people, invite by email, registration with approval or email domains

- Invites: link /?invite=CODE, copy link, send by email, max uses with counter
- Registration modes closed | approval | domain | domain+approval | open, waiting list with approve/reject
- REGISTRATION_MODE / REGISTRATION_DOMAINS lock the admin setting; ALLOW_REGISTRATION=true is honoured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/models.go

@@ -135,6 +135,8 @@ type User struct {
 	NotifyBeforeMinutes int      `json:"notifyBeforeMinutes"`
 	OrganizationID      string   `json:"organizationId,omitempty"`
 	CreatedAt           string   `json:"createdAt"`
+	// Registered while registration needs approval and not approved yet (admin user list only)
+	PendingApproval bool `json:"pendingApproval,omitempty"`
 	// Only filled for the user's own profile (GET /auth/me, PATCH /auth/settings)
 	NotificationSettings *NotificationSettings `json:"notificationSettings,omitempty"`
 }
@@ -216,7 +218,10 @@ type UserInvite struct {
 	CreatedBy        string `json:"createdBy"`
 	ExpiresAt        string `json:"expiresAt"`
 	UsedBy           string `json:"usedBy,omitempty"`
-	CreatedAt        string `json:"createdAt"`
+	// A link for several people: how often it may be used and how often it was
+	MaxUses   int    `json:"maxUses"`
+	UseCount  int    `json:"useCount"`
+	CreatedAt string `json:"createdAt"`
 }
 
 type ThoughtStatus string
@@ -350,12 +355,19 @@ type TeamMember struct {
 }
 
 type SystemSettings struct {
-	SharingMode       string `json:"sharingMode"` // private | organizations | open
-	AllowRegistration bool   `json:"allowRegistration"`
-	InstanceName      string `json:"instanceName"`
-	SMTPHost          string `json:"smtpHost"`
-	SMTPPort          string `json:"smtpPort"`
-	SMTPUser          string `json:"smtpUser"`
+	SharingMode string `json:"sharingMode"` // private | organizations | open
+	// AllowRegistration mirrors RegistrationMode == "open" (older clients and the setup wizard)
+	AllowRegistration bool `json:"allowRegistration"`
+	// Who may register without an invite: closed | approval | domain | domain+approval | open
+	RegistrationMode string `json:"registrationMode"`
+	// E-mail domains for the domain modes, comma separated ("firma.de, familie.de")
+	RegistrationDomains string `json:"registrationDomains"`
+	// Set by REGISTRATION_MODE / REGISTRATION_DOMAINS (or ALLOW_REGISTRATION=true): not editable here
+	RegistrationConfiguredViaEnv bool   `json:"registrationConfiguredViaEnv,omitempty"`
+	InstanceName                 string `json:"instanceName"`
+	SMTPHost                     string `json:"smtpHost"`
+	SMTPPort                     string `json:"smtpPort"`
+	SMTPUser                     string `json:"smtpUser"`
 	// SMTPPass is never sent to the browser (FLAW-34): reads leave it empty and set SMTPPassSet;
 	// writing it empty keeps the stored password
 	SMTPPass    string `json:"smtpPass"`