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>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "spoolman"
-version = "0.23C.4"
+version = "0.23C.5"
 description = "A web service that keeps track of 3D printing spools."
 authors = [
     { name = "Tony", email = "tony@nastynas.xyz" },