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>

app/views/base.py

@@ -32,6 +32,10 @@ def _inject_global_context(ctx: dict) -> None:
     ctx.setdefault("ui_default_color_scheme", getattr(settings, "ui_default_color_scheme", "system"))
     ctx.setdefault("multi_user_enabled", getattr(settings, "multi_user_enabled", False))
     ctx.setdefault("auth_enabled", getattr(settings, "auth_enabled", True))
+    ctx.setdefault(
+        "allow_signup",
+        getattr(settings, "multi_user_enabled", False) and getattr(settings, "allow_local_signup", False),
+    )
 
     req = ctx.get("request")
     if req is not None: