feat(admin): public-ticket guest policy fields in settings page

[mpge]

Summary

WordPress host-adapter side of the new public-ticket guest policy feature (escalated#32). WordPress doesn't use Inertia/Vue, so instead of a dedicated sub-page this extends the existing Escalated settings admin page with three new fields.

What's added

• `class-admin-settings.php` — `guest_policy_mode` (dropdown, sanitized string), `guest_policy_user_id` (absint), `guest_policy_signup_url_template` (esc_url_raw) added to the whitelisted settings fields.
• `templates/admin/settings.php` — new `` below the Guest Tickets checkbox with: mode dropdown, conditional shared-user-id number input, and signup URL template text input.
• Defensive post-processing rejects unknown modes (falls back to `unassigned`) and clears the two conditional fields when the mode doesn't apply, so the widget controller only ever sees settings that actually match the currently-selected mode.

Why inline instead of a new subpage

The shared `Admin/Settings/PublicTickets.vue` is Inertia-only — WordPress renders PHP templates. The existing `class-admin-settings.php` / `templates/admin/settings.php` pair already covers every other Escalated setting in one page, so adding three fields here is the minimal-surface way to deliver the feature.

Test plan

• Visit Escalated → Settings after merge, pick each of the three modes, verify the conditional fields only persist when their mode is active.
• Confirm `Escalated\Models\Setting::get('guest_policy_mode')` returns the stored value.