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.

backend/models/community/oauth_identity.js

@@ -0,0 +1,54 @@
+import { sequelize } from '../../utils/sequelize.js';
+import { DataTypes } from 'sequelize';
+
+const OAuthIdentity = sequelize.define('oauth_identity', {
+    userId: {
+        type: DataTypes.INTEGER,
+        allowNull: false,
+        field: 'user_id'
+    },
+    provider: {
+        type: DataTypes.STRING(64),
+        allowNull: false
+    },
+    issuer: {
+        type: DataTypes.TEXT,
+        allowNull: false
+    },
+    subject: {
+        type: DataTypes.TEXT,
+        allowNull: false
+    },
+    email: {
+        type: DataTypes.TEXT,
+        allowNull: true
+    },
+    createdAt: {
+        type: DataTypes.DATE,
+        allowNull: false,
+        field: 'created_at',
+        defaultValue: DataTypes.NOW
+    },
+    updatedAt: {
+        type: DataTypes.DATE,
+        allowNull: false,
+        field: 'updated_at',
+        defaultValue: DataTypes.NOW
+    }
+}, {
+    tableName: 'oauth_identity',
+    schema: 'community',
+    underscored: true,
+    timestamps: false,
+    indexes: [
+        {
+            unique: true,
+            fields: ['provider', 'subject']
+        },
+        {
+            fields: ['user_id']
+        }
+    ]
+});
+
+export default OAuthIdentity;