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>

.env.demo

@@ -133,6 +133,11 @@ ADMIN_GROUP_NAME=admin
 # When enabled, each user has their own document space with isolated uploads,
 # search, and file management. Requires AUTH_ENABLED=true.
 MULTI_USER_ENABLED=false
+# Allow users to self-register with an email address and password.
+# Set to true to enable the /signup page. Requires MULTI_USER_ENABLED=true.
+# When SMTP is configured, a verification email is sent before the account is activated.
+# Without SMTP, accounts are activated immediately upon registration.
+# ALLOW_LOCAL_SIGNUP=false
 # Default upload limit per user per day (0 = unlimited)
 DEFAULT_DAILY_UPLOAD_LIMIT=0
 # Show unowned documents (owner_id=NULL) to all users (true) or only admins (false)