Superadmin impersonation, hardened auth/upload paths, dependency updates

- Add superadmin impersonation: sessions.impersonated_by (migration 0003) is
  stamped onto audit rows so impersonated actions are attributable, with a
  persistent ImpersonationBanner in the app shell.
- Harden auth and upload handling across routes (safe redirect targets,
  filename sanitization, checkout grant handling).
- Update dependencies: Sentry 8 -> 10, @fastify/static 8 -> 10,
  react-router-dom 6.30.6; add find-my-way / fast-uri overrides.
- Add tests for safe-next, checkout-grant, and upload-filename.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/api/test/safe-next.test.ts

@@ -0,0 +1,79 @@
+import { describe, it, expect } from 'vitest';
+
+// Mirrors safeNext() in apps/web/src/pages/LoginPage.tsx — the post-login redirect guard. `next`
+// rides in the login URL and is therefore attacker-supplied, so an open redirect here lands a
+// freshly authenticated user on a phishing page. Lives in the API suite because the web workspace
+// has no test runner configured; keep the two copies in step.
+function safeNext(raw: string | null): string {
+  const FALLBACK = '/app';
+  if (!raw) return FALLBACK;
+  const cleaned = raw.replace(/[\t\n\r]/g, '');
+  if (!cleaned.startsWith('/') || cleaned.startsWith('//')) return FALLBACK;
+  try {
+    const probe = 'https://redirect-guard.invalid';
+    const url = new URL(cleaned, probe);
+    if (url.origin !== probe) return FALLBACK;
+    return `${url.pathname}${url.search}${url.hash}`;
+  } catch {
+    return FALLBACK;
+  }
+}
+
+const PROBE = 'https://redirect-guard.invalid';
+const staysInternal = (value: string) => new URL(value, PROBE).origin === PROBE;
+
+describe('safeNext — post-login open-redirect guard', () => {
+  it('passes through legitimate internal destinations', () => {
+    expect(safeNext('/app/cases')).toBe('/app/cases');
+    expect(safeNext('/app?tab=open#row')).toBe('/app?tab=open#row');
+    expect(safeNext('/admin')).toBe('/admin');
+  });
+
+  it('falls back when nothing was requested', () => {
+    expect(safeNext(null)).toBe('/app');
+    expect(safeNext('')).toBe('/app');
+  });
+
+  it('rejects absolute URLs to another origin', () => {
+    expect(safeNext('https://evil.com')).toBe('/app');
+    expect(safeNext('http://evil.com/path')).toBe('/app');
+  });
+
+  it('rejects protocol-relative URLs', () => {
+    expect(safeNext('//evil.com')).toBe('/app');
+    expect(safeNext('////evil.com')).toBe('/app');
+  });
+
+  it('rejects backslash variants that browsers normalise into an authority', () => {
+    // URL parsing rewrites \ as / for special schemes, so each of these would otherwise become
+    // protocol-relative and point off-origin. The origin check is what catches them.
+    for (const attack of ['/\\\\evil.com', '/\\/evil.com', '\\\\evil.com', '/\\\\\\evil.com']) {
+      expect(safeNext(attack)).toBe('/app');
+    }
+  });
+
+  it('rejects schemes smuggled past the leading-slash check with whitespace', () => {
+    expect(safeNext('\tjavascript:alert(1)')).toBe('/app');
+    expect(safeNext('\njavascript:alert(1)')).toBe('/app');
+    expect(safeNext('javascript:alert(1)')).toBe('/app');
+    expect(safeNext('data:text/html,<script>alert(1)</script>')).toBe('/app');
+  });
+
+  it('never returns a value that resolves off-origin', () => {
+    const attacks = [
+      '//evil.com',
+      '/\\evil.com',
+      '/\\\\evil.com',
+      'https://evil.com',
+      '\tjavascript:alert(1)',
+      '////evil.com',
+      '/%5cevil.com',
+      '/%2f%2fevil.com',
+      '/\t/evil.com',
+      'data:text/html,<script>alert(1)</script>',
+    ];
+    for (const attack of attacks) {
+      expect(staysInternal(safeNext(attack))).toBe(true);
+    }
+  });
+});