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.

internal/database/settings_registry.go

@@ -65,6 +65,7 @@ var SettingDefaults = []SettingDefault{
 	{Key: "password_require_lower", Value: "true", Type: SettingTypeBool, Category: "security", Group: "Password Quality", Description: "Require at least one lowercase letter (a-z)"},
 	{Key: "password_require_number", Value: "true", Type: SettingTypeBool, Category: "security", Group: "Password Quality", Description: "Require at least one number (0-9)"},
 	{Key: "password_require_special", Value: "true", Type: SettingTypeBool, Category: "security", Group: "Password Quality", Description: "Require at least one special character"},
+	{Key: "registration_enabled", Value: "true", Type: SettingTypeBool, Category: "security", Group: "Public Registration", Description: "Allow users to create their own accounts (admins can always create accounts)"},
 	{Key: "opds_default_page_size", Value: "50", Type: SettingTypeInt, Min: "1", Max: "500", Category: "api", Group: "OPDS Catalog", Description: "Default OPDS page size"},
 	{Key: "opds_max_page_size", Value: "200", Type: SettingTypeInt, Min: "1", Max: "1000", Category: "api", Group: "OPDS Catalog", Description: "Maximum OPDS page size"},
 	{Key: "device_rate_sync_per_min", Value: "60", Type: SettingTypeInt, Min: "1", Max: "10000", Category: "api", Group: "Device Rate Limits", Description: "Device sync requests per minute"},
@@ -188,6 +189,12 @@ func (r *SettingsRegistry) ScanPollInterval() time.Duration {
 // AutoScanEnabled reports whether auto-scanning is on.
 func (r *SettingsRegistry) AutoScanEnabled() bool { return r.getBool("auto_scan_enabled") }
 
+// RegistrationEnabled reports whether public signup is allowed. Admin user
+// creation is never affected — only self-service registration rides this
+// flag; handlers additionally grant the first-user exception (see
+// handlers/auth.go Register).
+func (r *SettingsRegistry) RegistrationEnabled() bool { return r.getBool("registration_enabled") }
+
 // DefaultTimezone returns the configured default timezone name.
 func (r *SettingsRegistry) DefaultTimezone() string { return r.raw("default_timezone") }