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/migrations-active/20260515000000-add-oauth-identities.cjs

@@ -0,0 +1,70 @@
+'use strict';
+
+/** @type {import('sequelize-cli').Migration} */
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable(
+      { schema: 'community', tableName: 'oauth_identity' },
+      {
+        id: {
+          type: Sequelize.INTEGER,
+          primaryKey: true,
+          autoIncrement: true,
+          allowNull: false,
+        },
+        user_id: {
+          type: Sequelize.INTEGER,
+          allowNull: false,
+          references: {
+            model: { schema: 'community', tableName: 'user' },
+            key: 'id',
+          },
+          onUpdate: 'CASCADE',
+          onDelete: 'CASCADE',
+        },
+        provider: {
+          type: Sequelize.STRING(64),
+          allowNull: false,
+        },
+        issuer: {
+          type: Sequelize.TEXT,
+          allowNull: false,
+        },
+        subject: {
+          type: Sequelize.TEXT,
+          allowNull: false,
+        },
+        email: {
+          type: Sequelize.TEXT,
+          allowNull: true,
+        },
+        created_at: {
+          type: Sequelize.DATE,
+          allowNull: false,
+          defaultValue: Sequelize.literal('CURRENT_TIMESTAMP'),
+        },
+        updated_at: {
+          type: Sequelize.DATE,
+          allowNull: false,
+          defaultValue: Sequelize.literal('CURRENT_TIMESTAMP'),
+        },
+      }
+    );
+
+    await queryInterface.addIndex(
+      { schema: 'community', tableName: 'oauth_identity' },
+      ['provider', 'subject'],
+      { unique: true, name: 'oauth_identity_provider_subject_uniq' }
+    );
+
+    await queryInterface.addIndex(
+      { schema: 'community', tableName: 'oauth_identity' },
+      ['user_id'],
+      { name: 'oauth_identity_user_idx' }
+    );
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable({ schema: 'community', tableName: 'oauth_identity' });
+  },
+};