Harden auth flows and add backend quality gate

frontend/app/admin/invites/page.tsx

@@ -156,6 +156,8 @@ const formatDate = (value?: string | null) => {
   return date.toLocaleString()
 }
 
+const isValidEmail = (value: string) => /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(value.trim())
+
 const isInviteTraceRowInvited = (row: InviteTraceRow) =>
   Boolean(String(row.inviterUsername || '').trim() || String(row.inviteCode || '').trim())
 
@@ -349,6 +351,17 @@ export default function AdminInviteManagementPage() {
 
   const saveInvite = async (event: React.FormEvent) => {
     event.preventDefault()
+    const recipientEmail = inviteForm.recipient_email.trim()
+    if (!recipientEmail) {
+      setError('Recipient email is required.')
+      setStatus(null)
+      return
+    }
+    if (!isValidEmail(recipientEmail)) {
+      setError('Recipient email must be valid.')
+      setStatus(null)
+      return
+    }
     setInviteSaving(true)
     setError(null)
     setStatus(null)
@@ -363,7 +376,7 @@ export default function AdminInviteManagementPage() {
         max_uses: inviteForm.max_uses || null,
         enabled: inviteForm.enabled,
         expires_at: inviteForm.expires_at || null,
-        recipient_email: inviteForm.recipient_email || null,
+        recipient_email: recipientEmail,
         send_email: inviteForm.send_email,
         message: inviteForm.message || null,
       }
@@ -1607,18 +1620,19 @@ export default function AdminInviteManagementPage() {
               <div className="invite-form-row">
                 <div className="invite-form-row-label">
                   <span>Delivery</span>
-                  <small>Save a recipient email and optionally send the invite immediately.</small>
+                  <small>Recipient email is required. You can optionally send the invite immediately after saving.</small>
                 </div>
                 <div className="invite-form-row-control invite-form-row-control--stacked">
                   <label>
-                    <span>Recipient email</span>
+                    <span>Recipient email (required)</span>
                     <input
                       type="email"
+                      required
                       value={inviteForm.recipient_email}
                       onChange={(e) =>
                         setInviteForm((current) => ({ ...current, recipient_email: e.target.value }))
                       }
-                      placeholder="person@example.com"
+                      placeholder="Required recipient email"
                     />
                   </label>
                   <label>