Add invite code registration system for beta

Backend:
- Add InviteCode model with single-use codes
- Add invite API endpoints (create, list, revoke, validate)
- Modify registration to require invite code when enabled
- Add REQUIRE_INVITE_CODE config toggle (default: true)
- Add Alembic migration for invite_codes table

Frontend:
- Add invite code field to registration page
- Validate invite code on blur with visual feedback
- Pass invite code to registration API

Admins can generate invite codes via /api/docs (Swagger UI).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -2,3 +2,4 @@ export { default as apiClient } from './client'
 export { default as authApi } from './auth'
 export { default as treesApi } from './trees'
 export { default as sessionsApi } from './sessions'
+export { default as inviteApi } from './invite'

frontend/src/api/invite.ts

@@ -0,0 +1,11 @@
+import apiClient from './client'
+import type { InviteCodeValidation } from '@/types'
+
+export const inviteApi = {
+  async validateCode(code: string): Promise<InviteCodeValidation> {
+    const response = await apiClient.get<InviteCodeValidation>(`/invites/validate/${code}`)
+    return response.data
+  },
+}
+
+export default inviteApi

frontend/src/pages/RegisterPage.tsx

@@ -1,23 +1,55 @@
 import { useState } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
 import { useAuthStore } from '@/store/authStore'
+import { inviteApi } from '@/api'
 import { cn } from '@/lib/utils'
 
 export function RegisterPage() {
   const navigate = useNavigate()
   const { register, isLoading, error, clearError } = useAuthStore()
 
+  const [inviteCode, setInviteCode] = useState('')
+  const [inviteCodeStatus, setInviteCodeStatus] = useState<'idle' | 'checking' | 'valid' | 'invalid'>('idle')
+  const [inviteCodeMessage, setInviteCodeMessage] = useState('')
   const [name, setName] = useState('')
   const [email, setEmail] = useState('')
   const [password, setPassword] = useState('')
   const [confirmPassword, setConfirmPassword] = useState('')
   const [localError, setLocalError] = useState('')
 
+  const validateInviteCode = async (code: string) => {
+    if (!code.trim()) {
+      setInviteCodeStatus('idle')
+      setInviteCodeMessage('')
+      return
+    }
+
+    setInviteCodeStatus('checking')
+    try {
+      const result = await inviteApi.validateCode(code.trim())
+      setInviteCodeStatus(result.valid ? 'valid' : 'invalid')
+      setInviteCodeMessage(result.message)
+    } catch {
+      setInviteCodeStatus('invalid')
+      setInviteCodeMessage('Failed to validate invite code')
+    }
+  }
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
     setLocalError('')
     clearError()
 
+    if (!inviteCode.trim()) {
+      setLocalError('Invite code is required')
+      return
+    }
+
+    if (inviteCodeStatus !== 'valid') {
+      setLocalError('Please enter a valid invite code')
+      return
+    }
+
     if (!name || !email || !password) {
       setLocalError('Please fill in all fields')
       return
@@ -34,7 +66,7 @@ export function RegisterPage() {
     }
 
     try {
-      await register({ email, password, name })
+      await register({ email, password, name, invite_code: inviteCode.trim() })
       navigate('/trees', { replace: true })
     } catch {
       // Error is set in the store
@@ -57,6 +89,43 @@ export function RegisterPage() {
               </div>
             )}
 
+            <div>
+              <label htmlFor="inviteCode" className="block text-sm font-medium text-foreground">
+                Invite code
+              </label>
+              <input
+                id="inviteCode"
+                name="inviteCode"
+                type="text"
+                required
+                value={inviteCode}
+                onChange={(e) => {
+                  setInviteCode(e.target.value.toUpperCase())
+                  setInviteCodeStatus('idle')
+                }}
+                onBlur={(e) => validateInviteCode(e.target.value)}
+                className={cn(
+                  'mt-1 block w-full rounded-md border bg-background px-3 py-2 font-mono tracking-wider',
+                  'text-foreground placeholder:text-muted-foreground',
+                  'focus:outline-none focus:ring-1',
+                  inviteCodeStatus === 'valid' && 'border-green-500 focus:border-green-500 focus:ring-green-500',
+                  inviteCodeStatus === 'invalid' && 'border-destructive focus:border-destructive focus:ring-destructive',
+                  inviteCodeStatus === 'idle' && 'border-input focus:border-primary focus:ring-primary',
+                  inviteCodeStatus === 'checking' && 'border-input focus:border-primary focus:ring-primary'
+                )}
+                placeholder="ABCD1234"
+              />
+              {inviteCodeStatus === 'checking' && (
+                <p className="mt-1 text-xs text-muted-foreground">Validating...</p>
+              )}
+              {inviteCodeStatus === 'valid' && (
+                <p className="mt-1 text-xs text-green-600">{inviteCodeMessage}</p>
+              )}
+              {inviteCodeStatus === 'invalid' && (
+                <p className="mt-1 text-xs text-destructive">{inviteCodeMessage}</p>
+              )}
+            </div>
+
             <div>
               <label htmlFor="name" className="block text-sm font-medium text-foreground">
                 Full name

frontend/src/types/index.ts

@@ -2,6 +2,7 @@ export * from './user'
 export * from './auth'
 export * from './tree'
 export * from './session'
+export * from './invite'
 
 // API response wrapper types
 export interface PaginatedResponse<T> {

frontend/src/types/invite.ts

@@ -0,0 +1,4 @@
+export interface InviteCodeValidation {
+  valid: boolean
+  message: string
+}

frontend/src/types/user.ts

@@ -15,6 +15,7 @@ export interface UserCreate {
   password: string
   name: string
   role?: UserRole
+  invite_code?: string
 }
 
 export interface UserLogin {