FIX: Correct OIDC cookie name mismatch across all auth endpoints

**Root Cause:**
- Auth system was looking for 'keycloak-session' cookies
- But actual OIDC system uses 'nuxt-oidc-auth' cookies
- This caused authentication failures for file previews and other endpoints

**Files Updated:**
- server/utils/auth.ts: Updated to check 'nuxt-oidc-auth' cookie
- server/api/auth/session.ts: Updated cookie name references
- server/api/auth/logout.ts: Updated cookie deletion
- server/api/auth/keycloak/callback.ts: Updated cookie creation

**Result:**
- File previews should now work for authenticated users
- All authentication endpoints now use consistent cookie names
- Both x-tag headers and OIDC sessions work correctly

server/utils/auth.ts

@@ -11,15 +11,15 @@ export const isAuthenticated = async (event: any): Promise<boolean> => {
     return true;
   }
 
-  // Check Keycloak session authentication
+  // Check OIDC session authentication
   try {
-    const keycloakSession = getCookie(event, 'keycloak-session');
-    if (keycloakSession) {
-      console.log('[auth] Authenticated via Keycloak session');
+    const oidcSession = getCookie(event, 'nuxt-oidc-auth');
+    if (oidcSession) {
+      console.log('[auth] Authenticated via OIDC session');
       return true;
     }
   } catch (error) {
-    console.log('[auth] Keycloak session check failed:', error);
+    console.log('[auth] OIDC session check failed:', error);
   }
 
   console.log('[auth] No valid authentication found');