Add OAuth integration for multiple providers and implement user linking

- Created OAuth credentials setup guide for Google, Microsoft, Keycloak, ORY, and ZITADEL.
- Added migration for oauth_identity table to store OAuth identities linked to users.
- Implemented OAuthIdentity model for managing OAuth identities in the database.
- Developed oauthService to handle OAuth login, user creation, and identity linking.
- Created OAuthCallbackView and OAuthUserCallbackView components for handling OAuth responses in the frontend.
- Added error handling and user feedback during the OAuth process.

frontend/src/router/authRoutes.js

@@ -1,4 +1,6 @@
 const ActivateView = () => import('../views/auth/ActivateView.vue');
+const OAuthCallbackView = () => import('../views/auth/OAuthCallbackView.vue');
+const OAuthUserCallbackView = () => import('../views/auth/OAuthUserCallbackView.vue');
 
 const authRoutes = [
     {
@@ -9,6 +11,22 @@ const authRoutes = [
             robots: 'noindex, nofollow'
         }
     },
+    {
+        path: '/auth/oauth/callback',
+        name: 'OAuth callback',
+        component: OAuthCallbackView,
+        meta: {
+            robots: 'noindex, nofollow'
+        }
+    },
+    {
+        path: '/auth/oauth/user/callback',
+        name: 'OAuth user callback',
+        component: OAuthUserCallbackView,
+        meta: {
+            robots: 'noindex, nofollow'
+        }
+    },
 ];
 
 export default authRoutes;