feat(auth): OAuth authorize and callback endpoints

Dynamic [provider] routes for Google, GitHub, and OIDC. Stores state and
PKCE verifier in httpOnly cookies. Callback validates code, finds/links
user by provider ID or email (invite-only), and creates session.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/server/auth.ts

@@ -266,6 +266,36 @@ export function updateUserRole(userId: number, role: 'admin' | 'user'): void {
 	db.update(users).set({ role }).where(eq(users.id, userId)).run();
 }
 
+/**
+ * Find a user by OAuth provider, or by email. Invite-only: rejects if no matching user.
+ */
+export function findOrLinkOAuthUser(
+	provider: string,
+	providerId: string,
+	email: string,
+	name: string
+): User | null {
+	// First try exact provider+providerId match
+	const byProvider = db
+		.select()
+		.from(users)
+		.where(and(eq(users.authProvider, provider), eq(users.providerId, providerId)))
+		.get();
+	if (byProvider) return toUser(byProvider);
+
+	// Try matching by email (invite-only: user must already exist)
+	const byEmail = db.select().from(users).where(eq(users.email, email)).get();
+	if (!byEmail) return null; // No matching user — not invited
+
+	// Link this OAuth provider to the existing user
+	db.update(users)
+		.set({ authProvider: provider, providerId, displayName: byEmail.displayName || name })
+		.where(eq(users.id, byEmail.id))
+		.run();
+
+	return toUser({ ...byEmail, authProvider: provider, providerId });
+}
+
 export function updateUserProfile(
 	userId: number,
 	data: { displayName?: string; email?: string }

src/routes/api/auth/oauth/[provider]/+server.ts

@@ -0,0 +1,65 @@
+import { redirect, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types.js';
+import { createGoogleAuthUrl } from '$lib/server/oauth/google.js';
+import { createGithubAuthUrl } from '$lib/server/oauth/github.js';
+import { createOidcAuthUrl } from '$lib/server/oauth/oidc.js';
+
+export const GET: RequestHandler = async ({ params, cookies }) => {
+	const { provider } = params;
+
+	if (provider === 'google') {
+		const result = createGoogleAuthUrl();
+		if (!result) throw error(404, 'Google OAuth not configured');
+
+		cookies.set('oauth_state', result.state, {
+			path: '/',
+			httpOnly: true,
+			sameSite: 'lax',
+			maxAge: 600
+		});
+		cookies.set('oauth_code_verifier', result.codeVerifier, {
+			path: '/',
+			httpOnly: true,
+			sameSite: 'lax',
+			maxAge: 600
+		});
+
+		throw redirect(302, result.url.toString());
+	}
+
+	if (provider === 'github') {
+		const result = createGithubAuthUrl();
+		if (!result) throw error(404, 'GitHub OAuth not configured');
+
+		cookies.set('oauth_state', result.state, {
+			path: '/',
+			httpOnly: true,
+			sameSite: 'lax',
+			maxAge: 600
+		});
+
+		throw redirect(302, result.url.toString());
+	}
+
+	if (provider === 'oidc') {
+		const result = await createOidcAuthUrl();
+		if (!result) throw error(404, 'OIDC not configured');
+
+		cookies.set('oauth_state', result.state, {
+			path: '/',
+			httpOnly: true,
+			sameSite: 'lax',
+			maxAge: 600
+		});
+		cookies.set('oauth_code_verifier', result.codeVerifier, {
+			path: '/',
+			httpOnly: true,
+			sameSite: 'lax',
+			maxAge: 600
+		});
+
+		throw redirect(302, result.url.toString());
+	}
+
+	throw error(404, 'Unknown provider');
+};

src/routes/api/auth/oauth/[provider]/callback/+server.ts

@@ -0,0 +1,63 @@
+import { redirect, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types.js';
+import { validateGoogleCallback } from '$lib/server/oauth/google.js';
+import { validateGithubCallback } from '$lib/server/oauth/github.js';
+import { validateOidcCallback } from '$lib/server/oauth/oidc.js';
+import { findOrLinkOAuthUser, createSession } from '$lib/server/auth.js';
+
+export const GET: RequestHandler = async ({ params, url, cookies, request, getClientAddress }) => {
+	const { provider } = params;
+	const code = url.searchParams.get('code');
+	const state = url.searchParams.get('state');
+	const storedState = cookies.get('oauth_state');
+
+	if (!code || !state || !storedState || state !== storedState) {
+		throw error(400, 'Invalid OAuth callback');
+	}
+
+	// Clean up OAuth cookies
+	cookies.delete('oauth_state', { path: '/' });
+
+	let profile: { email: string; name: string; providerId: string } | null = null;
+
+	if (provider === 'google') {
+		const codeVerifier = cookies.get('oauth_code_verifier');
+		cookies.delete('oauth_code_verifier', { path: '/' });
+		if (!codeVerifier) throw error(400, 'Missing code verifier');
+		profile = await validateGoogleCallback(code, codeVerifier);
+	} else if (provider === 'github') {
+		profile = await validateGithubCallback(code);
+	} else if (provider === 'oidc') {
+		const codeVerifier = cookies.get('oauth_code_verifier');
+		cookies.delete('oauth_code_verifier', { path: '/' });
+		if (!codeVerifier) throw error(400, 'Missing code verifier');
+		profile = await validateOidcCallback(code, codeVerifier);
+	} else {
+		throw error(404, 'Unknown provider');
+	}
+
+	if (!profile) {
+		throw error(400, 'Failed to get profile from OAuth provider');
+	}
+
+	// Find or link user (invite-only)
+	const user = findOrLinkOAuthUser(provider, profile.providerId, profile.email, profile.name);
+	if (!user) {
+		// User not found — not invited
+		throw redirect(302, '/login?error=no_account');
+	}
+
+	// Create session
+	const userAgent = request.headers.get('user-agent') || undefined;
+	const ip = getClientAddress();
+	const token = await createSession(user.id, { userAgent, ip });
+
+	cookies.set('session', token, {
+		path: '/',
+		httpOnly: true,
+		sameSite: 'lax',
+		maxAge: 30 * 24 * 60 * 60
+	});
+
+	throw redirect(302, '/');
+};