Make auth toggle a settings option instead of env var

- Auth enabled state now stored in database setting (auth_enabled)
- Add POST /auth/toggle endpoint to enable/disable auth from UI
- Add auth toggle section to Settings > General page
- Enabling auth without users prompts to create first admin
- Disabling auth requires admin password confirmation
- Auth setting cached in memory and loaded on startup

Removes need for SPOOLMAN_AUTH_ENABLED env var

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/public/locales/en/common.json

@@ -390,6 +390,22 @@
             "replace_warning_title": "Replace all data?",
             "replace_warning": "This will DELETE all existing data and replace it with the backup. This cannot be undone!"
         },
+        "auth": {
+            "title": "Authentication",
+            "description": "Require users to log in to access Spoolman. Once enabled, you'll be redirected to the login page.",
+            "status_enabled": "Authentication is enabled",
+            "status_disabled": "Authentication is disabled",
+            "setup_title": "Enable Authentication",
+            "setup_description": "Create an admin account to enable authentication. This user will have full access to manage Spoolman.",
+            "enable_button": "Create Admin & Enable",
+            "disable_title": "Disable Authentication",
+            "disable_description": "Enter an admin password to confirm disabling authentication. Anyone will be able to access Spoolman without logging in.",
+            "disable_button": "Disable Authentication",
+            "admin_password": "Admin Password",
+            "password_required": "Password is required",
+            "enabled_success": "Authentication enabled",
+            "disabled_success": "Authentication disabled"
+        },
         "extra_fields": {
             "tab": "Extra Fields",
             "description": "<p>Here you can add extra custom fields to your entities.</p><p>Once a field is added, you can not change its key or type, and for choice type fields you can not remove choices or change the multi choice state. If you remove a field, the associated data for all entities will be deleted.</p><p>The key is what other programs read/write the data as, so if your custom field is supposed to integrate with a third-party program, make sure to set it correctly. Default value is only applied to new items.</p><p>Extra fields can not be sorted or filtered in the table views.</p>",
@@ -449,7 +465,7 @@
             "role_editor": "Editor - Can view, use, create, and edit",
             "role_admin": "Admin - Full access including user management",
             "auth_disabled": "Authentication is not enabled",
-            "auth_disabled_hint": "Set SPOOLMAN_AUTH_ENABLED=true to enable user management",
+            "auth_disabled_hint": "Enable authentication in Settings > General to manage users",
             "admin_required": "Admin Access Required",
             "admin_required_hint": "Only administrators can manage users",
             "logout": "Log Out"