feat: registration_enabled admin setting — hides signup surfaces, gates the API

One boolean (Security > Public Registration) switches the whole public
signup lifecycle: front page + login links, the logged-out sidebar's
Create-an-account (via a package-level templates hook so shared page
templates keep their signatures), GET /register -> 302 /login, and an
early 403 on POST /api/auth/register. First-user exception preserved:
zero admins keeps every route and link reachable for bootstrapping —
same 'users exist' reasoning as the setup gate. Admin user creation is
unaffected by design.

Live E2E verified: flag off hides all surfaces and blocks the POST
(403), flag on restores them (201); the admin UI renders the row
automatically.

database/schema/schema.sql

@@ -68,6 +68,7 @@ INSERT INTO system_settings (setting_key, setting_value, description, setting_ty
 ('password_require_lower', 'true', 'Require at least one lowercase letter (a-z)', 'bool', NULL, NULL, FALSE, 'security'),
 ('password_require_number', 'true', 'Require at least one number (0-9)', 'bool', NULL, NULL, FALSE, 'security'),
 ('password_require_special', 'true', 'Require at least one special character', 'bool', NULL, NULL, FALSE, 'security'),
+('registration_enabled', 'true', 'Allow users to create their own accounts (admins can always create accounts)', 'bool', NULL, NULL, FALSE, 'security'),
 -- security / auth (restart required)
 ('auth_rate_limit_per_min', '10', 'Global auth API rate limit (requests per minute)', 'int', '1', '10000', TRUE, 'security'),
 ('login_max_attempts', '5', 'Failed login attempts before lockout', 'int', '1', '100', TRUE, 'security'),