feat(observability): add handleError callback to hooks.server.ts returning errorId

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

frontend/src/hooks.server.test.ts

@@ -0,0 +1,58 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+vi.mock('@sentry/sveltekit', () => ({
+	init: vi.fn(),
+	handleErrorWithSentry: (fn: (args: unknown) => unknown) => fn,
+	lastEventId: vi.fn(() => 'sentry-event-id-abc123')
+}));
+
+vi.mock('@sveltejs/kit', () => ({ redirect: vi.fn() }));
+vi.mock('@sveltejs/kit/hooks', () => ({ sequence: vi.fn((...fns: unknown[]) => fns[0]) }));
+vi.mock('$lib/paraglide/server', () => ({ paraglideMiddleware: vi.fn() }));
+vi.mock('$lib/paraglide/runtime', () => ({ cookieName: 'locale', cookieMaxAge: 86400 }));
+vi.mock('$lib/shared/server/locale', () => ({ detectLocale: vi.fn(() => 'de') }));
+
+const makeEvent = () => ({
+	url: { pathname: '/documents/123' },
+	locals: {}
+});
+
+describe('hooks.server handleError', () => {
+	beforeEach(() => {
+		vi.resetModules();
+	});
+
+	it('returns Sentry lastEventId as errorId', async () => {
+		const Sentry = await import('@sentry/sveltekit');
+		vi.mocked(Sentry.lastEventId).mockReturnValue('sentry-event-id-abc123');
+
+		const { handleError } = await import('./hooks.server');
+		const result = (handleError as (args: unknown) => { message: string; errorId: string })({
+			error: new Error('boom'),
+			event: makeEvent(),
+			status: 500,
+			message: 'Internal Error'
+		});
+
+		expect(result.errorId).toBe('sentry-event-id-abc123');
+		expect(result.message).toBe('An unexpected error occurred');
+	});
+
+	it('falls back to crypto.randomUUID when lastEventId returns undefined', async () => {
+		const Sentry = await import('@sentry/sveltekit');
+		vi.mocked(Sentry.lastEventId).mockReturnValue(undefined);
+
+		const { handleError } = await import('./hooks.server');
+		const result = (handleError as (args: unknown) => { message: string; errorId: string })({
+			error: new Error('boom'),
+			event: makeEvent(),
+			status: 500,
+			message: 'Internal Error'
+		});
+
+		expect(result.errorId).toMatch(
+			/^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/
+		);
+		expect(result.message).toBe('An unexpected error occurred');
+	});
+});