feat(auth): add "Continue with Google" OAuth 2.0 login

Server-side authorization-code flow: /v1/auth/google redirects to the
consent screen with a CSRF state cookie; /v1/auth/google/callback
exchanges the code, validates the ID token (iss/aud/exp/email_verified),
and mints a 30-day session via upsertOAuthLogin. /v1/auth/providers lets
the login UI hide the button until GOOGLE_OAUTH_ID/SECRET are set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/api/src/routes/auth.ts

@@ -1,16 +1,50 @@
-import type { FastifyInstance } from 'fastify';
-import { z } from 'zod';
+import crypto from 'node:crypto';
 import {
   consumeMagicLink,
   destroySession,
   getSession,
   issueMagicLink,
   loginWithPassword,
+  upsertOAuthLogin,
 } from '@bmm/auth';
-import { audit } from '../lib/audit.js';
+import type { FastifyInstance } from 'fastify';
+import { z } from 'zod';
 import { config } from '../config.js';
+import { audit } from '../lib/audit.js';
 
 const SESSION_COOKIE = 'bmm_session';
+const OAUTH_STATE_COOKIE = 'bmm_oauth_state';
+
+const GoogleClaims = z.object({
+  iss: z.string(),
+  aud: z.string(),
+  exp: z.number(),
+  email: z.string().email(),
+  email_verified: z.union([z.boolean(), z.string()]).optional(),
+  name: z.string().optional(),
+});
+
+/**
+ * Decode (NOT signature-verify) a Google ID token payload. Signature verification
+ * is unnecessary here because the token is fetched directly from Google's token
+ * endpoint over TLS, authenticated with our client secret — an intermediary-free
+ * channel, per Google's own guidance. We still validate iss / aud / exp / email
+ * below as defense-in-depth.
+ */
+function decodeGoogleIdToken(idToken: string): z.infer<typeof GoogleClaims> {
+  const parts = idToken.split('.');
+  if (parts.length !== 3 || !parts[1]) throw new Error('malformed_id_token');
+  const json = Buffer.from(parts[1], 'base64url').toString('utf8');
+  return GoogleClaims.parse(JSON.parse(json));
+}
+
+function googleRedirectUri(): string {
+  return `${config.CONTROL_PLANE_PUBLIC_URL}/v1/auth/google/callback`;
+}
+
+function googleConfigured(): boolean {
+  return Boolean(config.GOOGLE_OAUTH_ID && config.GOOGLE_OAUTH_SECRET);
+}
 
 export async function authRoutes(app: FastifyInstance): Promise<void> {
   app.post('/v1/auth/magic-link', async (req, reply) => {
@@ -132,4 +166,114 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
     }
     return reply.send({ ok: true });
   });
+
+  // Which third-party login providers are configured. Lets the UI hide the
+  // Google button when no credentials are set, instead of showing a dead button.
+  app.get('/v1/auth/providers', async (_req, reply) => {
+    return reply.send({ google: googleConfigured() });
+  });
+
+  // Step 1: hand the browser off to Google's consent screen.
+  app.get('/v1/auth/google', async (_req, reply) => {
+    if (!config.GOOGLE_OAUTH_ID || !config.GOOGLE_OAUTH_SECRET) {
+      return reply.code(503).send({ error: 'google_oauth_not_configured' });
+    }
+    const state = crypto.randomBytes(16).toString('base64url');
+    reply.setCookie(OAUTH_STATE_COOKIE, state, {
+      httpOnly: true,
+      sameSite: 'lax',
+      path: '/',
+      secure: config.NODE_ENV === 'production',
+      maxAge: 600,
+    });
+    const url = new URL('https://accounts.google.com/o/oauth2/v2/auth');
+    url.searchParams.set('client_id', config.GOOGLE_OAUTH_ID);
+    url.searchParams.set('redirect_uri', googleRedirectUri());
+    url.searchParams.set('response_type', 'code');
+    url.searchParams.set('scope', 'openid email profile');
+    url.searchParams.set('state', state);
+    url.searchParams.set('access_type', 'online');
+    url.searchParams.set('prompt', 'select_account');
+    return reply.redirect(url.toString());
+  });
+
+  // Step 2: Google redirects back here with an auth code. Exchange it, verify
+  // the ID token, mint a session, drop the user on the dashboard.
+  app.get('/v1/auth/google/callback', async (req, reply) => {
+    const loginUrl = `${config.NEXT_PUBLIC_APP_URL}/login`;
+    const Query = z.object({
+      code: z.string().min(10).optional(),
+      state: z.string().min(8).optional(),
+      error: z.string().optional(),
+    });
+    const q = Query.safeParse(req.query);
+    const cookieState = req.cookies[OAUTH_STATE_COOKIE];
+    reply.clearCookie(OAUTH_STATE_COOKIE, { path: '/' });
+
+    if (!q.success || q.data.error || !q.data.code || !q.data.state) {
+      return reply.redirect(`${loginUrl}?error=google_failed`);
+    }
+    // CSRF: the state echoed back by Google must match the one we set.
+    // Length-check first — timingSafeEqual throws on a length mismatch.
+    if (
+      !cookieState ||
+      cookieState.length !== q.data.state.length ||
+      !crypto.timingSafeEqual(Buffer.from(cookieState), Buffer.from(q.data.state))
+    ) {
+      return reply.redirect(`${loginUrl}?error=google_state`);
+    }
+    if (!config.GOOGLE_OAUTH_ID || !config.GOOGLE_OAUTH_SECRET) {
+      return reply.redirect(`${loginUrl}?error=google_failed`);
+    }
+
+    try {
+      const tokenRes = await fetch('https://oauth2.googleapis.com/token', {
+        method: 'POST',
+        headers: { 'content-type': 'application/x-www-form-urlencoded' },
+        body: new URLSearchParams({
+          code: q.data.code,
+          client_id: config.GOOGLE_OAUTH_ID,
+          client_secret: config.GOOGLE_OAUTH_SECRET,
+          redirect_uri: googleRedirectUri(),
+          grant_type: 'authorization_code',
+        }),
+      });
+      if (!tokenRes.ok) throw new Error(`token_exchange_${tokenRes.status}`);
+      const tokens = (await tokenRes.json()) as { id_token?: string };
+      if (!tokens.id_token) throw new Error('no_id_token');
+
+      const claims = decodeGoogleIdToken(tokens.id_token);
+      if (claims.iss !== 'accounts.google.com' && claims.iss !== 'https://accounts.google.com') {
+        throw new Error('bad_iss');
+      }
+      if (claims.aud !== config.GOOGLE_OAUTH_ID) throw new Error('bad_aud');
+      if (claims.exp * 1000 < Date.now()) throw new Error('token_expired');
+      const verified = claims.email_verified === true || claims.email_verified === 'true';
+      if (!verified) throw new Error('email_unverified');
+
+      const session = await upsertOAuthLogin(
+        { email: claims.email, name: claims.name ?? null },
+        { ipAddress: req.ip, userAgent: req.headers['user-agent'] },
+      );
+      reply.setCookie(SESSION_COOKIE, session.sessionToken, {
+        httpOnly: true,
+        sameSite: 'lax',
+        path: '/',
+        secure: config.NODE_ENV === 'production',
+        maxAge: 30 * 24 * 60 * 60,
+      });
+      await audit({
+        orgId: session.orgId,
+        userId: session.userId,
+        action: 'auth.login',
+        resourceType: 'session',
+        metadata: { email: session.email, provider: 'google' },
+        ipAddress: req.ip,
+      });
+      return reply.redirect(`${config.NEXT_PUBLIC_APP_URL}/dashboard`);
+    } catch (err) {
+      app.log.warn({ err }, 'google oauth callback failed');
+      return reply.redirect(`${loginUrl}?error=google_failed`);
+    }
+  });
 }