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>

spoolman/main.py

@@ -13,7 +13,7 @@ from fastapi.responses import PlainTextResponse, RedirectResponse, Response
 from prometheus_client import generate_latest
 from scheduler.asyncio.scheduler import Scheduler
 
-from spoolman import env, externaldb
+from spoolman import auth, env, externaldb
 from spoolman.api.v1.router import app as v1_app
 from spoolman.client import SinglePageApplication
 from spoolman.database import database
@@ -173,6 +173,12 @@ async def startup() -> None:
     project_root = Path(__file__).parent.parent
     subprocess.run(["alembic", "upgrade", "head"], check=True, cwd=project_root)  # noqa: S603, S607, ASYNC221
 
+    # Load auth enabled setting from database
+    logger.info("Loading auth settings...")
+    async for db in database.get_db_session():
+        auth_enabled = await auth.load_auth_enabled_from_db(db)
+        logger.info("Authentication enabled: %s", auth_enabled)
+
     # Setup scheduler
     schedule = Scheduler()
     database.schedule_tasks(schedule)