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/controllers/authController.js

@@ -1,4 +1,5 @@
 import * as userService from '../services/authService.js';
+import * as oauthService from '../services/oauthService.js';
 
 class AuthController {
     constructor() {
@@ -7,6 +8,13 @@ class AuthController {
         this.forgotPassword = this.forgotPassword.bind(this);
         this.activateAccount = this.activateAccount.bind(this);
         this.logout = this.logout.bind(this);
+        this.oauthProviders = this.oauthProviders.bind(this);
+        this.oauthStart = this.oauthStart.bind(this);
+        this.oauthExchange = this.oauthExchange.bind(this);
+        this.oauthUserIdentities = this.oauthUserIdentities.bind(this);
+        this.oauthUserStart = this.oauthUserStart.bind(this);
+        this.oauthUserExchange = this.oauthUserExchange.bind(this);
+        this.oauthUserRemove = this.oauthUserRemove.bind(this);
     }
 
     async register(req, res) {
@@ -43,6 +51,133 @@ class AuthController {
         res.status(200).json({ result: 'loggedout' });
     }
 
+    async oauthProviders(req, res) {
+        try {
+            const providers = await oauthService.getOAuthProviders();
+            res.status(200).json({ providers });
+        } catch (error) {
+            res.status(500).json({ error: error.message });
+        }
+    }
+
+    async oauthStart(req, res) {
+        const { provider } = req.params;
+        try {
+            const redirectTo = await oauthService.startOAuthLogin({ providerSlug: provider });
+            res.redirect(302, redirectTo.toString());
+        } catch (error) {
+            const status = error.message === 'providernotconfigured' ? 503 : 500;
+            res.status(status).json({ error: error.message });
+        }
+    }
+
+    async oauthExchange(req, res) {
+        const { code, state } = req.body;
+        try {
+            const result = await oauthService.exchangeOAuthLogin({ code, state });
+            res.status(200).json(result);
+        } catch (error) {
+            const knownErrors = new Set([
+                'oauthcodemissing',
+                'oauthstatemissing',
+                'oauthsubjectmissing',
+                'providernotconfigured',
+                'oauthidentityconflict'
+            ]);
+            const status = knownErrors.has(error.message) ? 400 : 500;
+            res.status(status).json({ error: error.message });
+        }
+    }
+
+    async oauthUserIdentities(req, res) {
+        const { userid: hashedUserId } = req.headers;
+        try {
+            const User = (await import('../models/community/user.js')).default;
+            const user = await User.findOne({ where: { hashedId: hashedUserId } });
+            if (!user) {
+                return res.status(404).json({ error: 'usernotfound' });
+            }
+
+            const identities = await oauthService.getUserOAuthIdentities(user.id);
+            res.status(200).json({ identities });
+        } catch (error) {
+            res.status(500).json({ error: error.message });
+        }
+    }
+
+    async oauthUserStart(req, res) {
+        const { userid: hashedUserId } = req.headers;
+        const { provider } = req.params;
+        try {
+            const User = (await import('../models/community/user.js')).default;
+            const user = await User.findOne({ where: { hashedId: hashedUserId } });
+            if (!user) {
+                return res.status(404).json({ error: 'usernotfound' });
+            }
+
+            const redirectTo = await oauthService.startOAuthLoginForUser({
+                userId: user.id,
+                providerSlug: provider
+            });
+            res.redirect(302, redirectTo.toString());
+        } catch (error) {
+            const status = error.message === 'providernotconfigured' ? 503 : 500;
+            res.status(status).json({ error: error.message });
+        }
+    }
+
+    async oauthUserExchange(req, res) {
+        const { userid: hashedUserId } = req.headers;
+        const { code, state } = req.body;
+        try {
+            const User = (await import('../models/community/user.js')).default;
+            const user = await User.findOne({ where: { hashedId: hashedUserId } });
+            if (!user) {
+                return res.status(404).json({ error: 'usernotfound' });
+            }
+
+            const result = await oauthService.exchangeOAuthLoginForUser({
+                userId: user.id,
+                code,
+                state
+            });
+            res.status(200).json(result);
+        } catch (error) {
+            const knownErrors = new Set([
+                'oauthcodemissing',
+                'oauthstatemissing',
+                'oauthsubjectmissing',
+                'providernotconfigured',
+                'oauthuseridmismatch',
+                'oauthidentityalreadylinked'
+            ]);
+            const status = knownErrors.has(error.message) ? 400 : 500;
+            res.status(status).json({ error: error.message });
+        }
+    }
+
+    async oauthUserRemove(req, res) {
+        const { userid: hashedUserId } = req.headers;
+        const { identityId } = req.params;
+        try {
+            const User = (await import('../models/community/user.js')).default;
+            const user = await User.findOne({ where: { hashedId: hashedUserId } });
+            if (!user) {
+                return res.status(404).json({ error: 'usernotfound' });
+            }
+
+            const result = await oauthService.removeOAuthIdentity({
+                userId: user.id,
+                identityId: parseInt(identityId, 10)
+            });
+            res.status(200).json(result);
+        } catch (error) {
+            const status = error.message === 'forbidden' ? 403 : 500;
+            res.status(status).json({ error: error.message });
+        }
+    }
+
+
     async forgotPassword(req, res) {
         const { email } = req.body;
         try {