feat(auth): enable local user signup without SMTP, add admin user creation

- Remove SMTP hard-requirement from /api/auth/signup: when SMTP is not
  configured accounts are activated immediately (no email verification).
  When SMTP is configured the existing email-verification flow is kept.
- Inject allow_signup into global template context via app/views/base.py
- Add data-allow-signup attribute to base.html body tag
- Update common.js _renderLoggedOutAuth to show Sign Up (→ /signup) when
  signup is enabled, otherwise Get Started (→ /pricing)
- Add admin API endpoints before the /{user_id:path} catch-all:
    GET  /api/admin/users/local       – list all local accounts
    POST /api/admin/users/local       – admin-create local account (active immediately)
    DELETE /api/admin/users/local/{id} – delete local account + profile
- Add LocalUserCreate / LocalUserResponse Pydantic schemas
- Update admin_users.html with Local User Accounts section and modals
- Update .env.demo to document ALLOW_LOCAL_SIGNUP
- Update docs/BillingSetup.md: SMTP is optional, document both flows
- Update tests: test_signup_smtp_not_configured now asserts 201 + immediate
  activation; add 7 new integration tests for admin local user endpoints

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/signup.html

@@ -50,7 +50,12 @@
                             })
                         });
                         if (resp.ok) {
-                            window.location.href = '/verify-email-sent';
+                            const data = await resp.json();
+                            if (data.email_verification_required) {
+                                window.location.href = '/verify-email-sent';
+                            } else {
+                                window.location.href = '/login?message=Account+created+successfully.+You+can+now+log+in.';
+                            }
                         } else {
                             const data = await resp.json();
                             this.error = data.detail || 'Registration failed. Please try again.';