feat(auth): split OIDC_ONLY into granular auth toggles

Replaces the coarse oidc_only + allow_registration settings with four
independent toggles: password_login, password_registration, oidc_login,
oidc_registration. Each can be enabled/disabled individually in
Admin > Settings without affecting the others.

- Add resolveAuthToggles() in authService.ts as the central resolver;
  falls back to legacy oidc_only/allow_registration keys when new keys
  are absent (backward compat)
- OIDC_ONLY env var still works and overrides DB toggles for password_*,
  with a visual lock in the admin UI when active
- Server enforces lockout prevention: cannot disable all login methods
- oidc_login gate added to OIDC /login and /callback routes
- Remove oidc_only toggle from OIDC settings panel; replaced by the
  granular toggles in the Settings tab
- Add 6 new resolveAuthToggles() unit tests; fix AUTH-DB-033 error
  message assertion
- Update OIDC_ONLY descriptions in README, docker-compose, Helm values,
  Unraid template, and .env.example to clarify override semantics

Closes #492

server/src/routes/oidc.ts

@@ -15,12 +15,17 @@ import {
   frontendUrl,
   getAppUrl,
 } from '../services/oidcService';
+import { resolveAuthToggles } from '../services/authService';
 
 const router = express.Router();
 
 // ---- GET /login ----------------------------------------------------------
 
 router.get('/login', async (req: Request, res: Response) => {
+  if (!resolveAuthToggles().oidc_login) {
+    return res.status(403).json({ error: 'SSO login is disabled.' });
+  }
+
   const config = getOidcConfig();
   if (!config) return res.status(400).json({ error: 'OIDC not configured' });
 
@@ -57,6 +62,10 @@ router.get('/login', async (req: Request, res: Response) => {
 // ---- GET /callback -------------------------------------------------------
 
 router.get('/callback', async (req: Request, res: Response) => {
+  if (!resolveAuthToggles().oidc_login) {
+    return res.redirect(frontendUrl('/login?oidc_error=sso_disabled'));
+  }
+
   const { code, state, error: oidcError } = req.query as { code?: string; state?: string; error?: string };
 
   if (oidcError) {