feat(auth): add /accept-invite page + lookup endpoint

Adds the invitee-side flow for self-serve signup Phase 2 (Task 36):

Backend
- Public GET /accounts/invites/{code}/lookup returns
  {account_name, inviter_name, invited_email, role} for a valid invite,
  404 invite_invalid_or_expired_or_revoked otherwise (collapses unknown /
  expired / revoked / used into one anti-enumeration response). Mounted
  in a new account_invite_lookup endpoints module on the public route
  list, uses get_admin_db (BYPASSRLS) since the caller has no tenant.
- OAuthCallbackPayload gains optional account_invite_code + invited_email.
  _sign_in_or_register honors them: a new OAuth user with a valid invite
  joins the invited account (no personal account, no Pro trial), the
  invite is marked used, and OAuth-profile-email vs invite-email mismatch
  raises invite_email_mismatch (matching the email+password register
  contract).

Frontend
- New public route /accept-invite -> AcceptInvitePage. Reads ?code=,
  calls inviteApi.lookupAccountInvite, renders "Join {account} on
  ResolutionFlow" with the invited email locked (rendered as a div, not
  an input), three sign-in options (set password, Google, Microsoft),
  and a clear "ask {inviter} to resend" + mailto: fallback for invalid
  codes.
- OAuth state for invitees is base64url(JSON({csrf, accountInviteCode,
  invitedEmail})). OAuthCallbackPage decodes both shapes, forwards the
  invite fields to the backend, and surfaces invite_email_mismatch /
  invite_invalid_or_expired_or_revoked errors with friendly text.
  Successful invite-OAuth lands on /?welcome=teammate (suppresses the
  welcome wizard for invitees per spec).
- UserCreate type + invite/auth API clients extended for the new fields.

Tests
- Backend: invite lookup happy path + four invalid-state collapse, OAuth
  callback links invite when supplied + rejects on email mismatch.
- Frontend Vitest: AcceptInvitePage renders account name + locked email
  + accept buttons; resend message + mailto on invalid code.

All 43 backend auth/account/invite/email-verification tests green;
frontend Vitest 120/120 green; tsc -b clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/__tests__/AcceptInvitePage.test.tsx

@@ -0,0 +1,123 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { render, screen, waitFor } from '@testing-library/react'
+import { MemoryRouter } from 'react-router-dom'
+import { HelmetProvider } from 'react-helmet-async'
+
+import { AcceptInvitePage } from '../AcceptInvitePage'
+import { inviteApi } from '@/api/invite'
+import {
+  __resetAppConfigCache,
+  __setAppConfigCache,
+} from '@/hooks/useAppConfig'
+
+vi.mock('@/api/invite', () => ({
+  inviteApi: {
+    lookupAccountInvite: vi.fn(),
+    validateCode: vi.fn(),
+  },
+}))
+
+vi.mock('@/store/authStore', () => ({
+  useAuthStore: () => ({
+    register: vi.fn().mockResolvedValue(undefined),
+    isLoading: false,
+    error: null,
+    clearError: vi.fn(),
+  }),
+}))
+
+function renderPage(initialPath: string) {
+  return render(
+    <HelmetProvider>
+      <MemoryRouter initialEntries={[initialPath]}>
+        <AcceptInvitePage />
+      </MemoryRouter>
+    </HelmetProvider>,
+  )
+}
+
+describe('AcceptInvitePage', () => {
+  beforeEach(() => {
+    __resetAppConfigCache()
+    __setAppConfigCache({
+      self_serve_enabled: true,
+      oauth_providers: ['google', 'microsoft'],
+    })
+    vi.clearAllMocks()
+  })
+
+  it('shows account name + locked email + accept buttons for a valid code', async () => {
+    vi.mocked(inviteApi.lookupAccountInvite).mockResolvedValue({
+      account_name: 'Acme MSP',
+      inviter_name: 'Alice Owner',
+      invited_email: 'bob@acme.example',
+      role: 'engineer',
+    })
+
+    renderPage('/accept-invite?code=VALIDINVITECODE0011223344556677')
+
+    // Inviter context (also confirms the lookup completed and rendered)
+    await waitFor(() => {
+      expect(
+        screen.getByText(/Alice Owner invited you as engineer/),
+      ).toBeInTheDocument()
+    })
+    // Account name surfaces in the heading line.
+    expect(
+      screen.getByText((_content, node) => {
+        return (
+          node?.tagName.toLowerCase() === 'span' &&
+          /Acme MSP/.test(node.textContent || '')
+        )
+      }),
+    ).toBeInTheDocument()
+
+    // Locked email — not an editable input
+    const emailDisplay = screen.getByTestId('invited-email')
+    expect(emailDisplay.tagName.toLowerCase()).not.toBe('input')
+    expect(emailDisplay).toHaveTextContent('bob@acme.example')
+    expect(screen.queryByLabelText(/email address/i)).not.toBeInTheDocument()
+
+    // OAuth buttons + password submit all rendered
+    expect(screen.getByTestId('oauth-google')).toBeInTheDocument()
+    expect(screen.getByTestId('oauth-microsoft')).toBeInTheDocument()
+    expect(screen.getByTestId('accept-submit')).toBeInTheDocument()
+    expect(screen.getByTestId('accept-submit')).toHaveTextContent(/Join Acme MSP/)
+
+    expect(inviteApi.lookupAccountInvite).toHaveBeenCalledWith(
+      'VALIDINVITECODE0011223344556677',
+    )
+  })
+
+  it('shows resend message + mailto link for an invalid invite code', async () => {
+    vi.mocked(inviteApi.lookupAccountInvite).mockRejectedValue(
+      Object.assign(new Error('not found'), {
+        response: {
+          status: 404,
+          data: { detail: { error: 'invite_invalid_or_expired_or_revoked' } },
+        },
+      }),
+    )
+
+    renderPage('/accept-invite?code=BADCODE')
+
+    await waitFor(() => {
+      expect(
+        screen.getByText(/This invite is no longer valid/i),
+      ).toBeInTheDocument()
+    })
+    expect(
+      screen.getByText(/Ask the person who invited you to resend it/i),
+    ).toBeInTheDocument()
+
+    const resendLink = screen.getByRole('link', { name: /Email your inviter/i })
+    expect(resendLink).toHaveAttribute(
+      'href',
+      expect.stringMatching(/^mailto:/),
+    )
+
+    // No accept form rendered when invite is invalid.
+    expect(screen.queryByTestId('accept-submit')).not.toBeInTheDocument()
+    expect(screen.queryByTestId('oauth-google')).not.toBeInTheDocument()
+  })
+})