feat(security): harden canvas server with auth, rate-limiting, and validation

- Add security.ts: helmet, CORS allowlist, timing-safe API key auth, prototype
  pollution guard, Mermaid input limits, rate limiting (general/destructive/burst)
- WS auth challenge-response with 5 s timeout and close code 4001
- Fix sync crash: array check before logger access (500 → 400)
- Fix sync/v2: validate element type before write (invalid → 400)
- Upgrade zod 3.22.4 → 3.25.5 (fixes ERR_PACKAGE_PATH_NOT_EXPORTED on startup)
- Extract ElementSharedFieldsSchema; move VALID_ELEMENT_TYPES to module level
- Docker: resource limits, .dockerignore hardening
- Add .project-hooks/pre-commit; expand test coverage (369 tests)

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

tests/backend/ws.test.ts

@@ -198,7 +198,7 @@ describe('WebSocket broadcasts', () => {
 
     const clearedPromise = waitForMessageOfType(ws, 'canvas_cleared');
 
-    await fetch(`http://localhost:${port}/api/elements/clear`, { method: 'DELETE' });
+    await fetch(`http://localhost:${port}/api/elements/clear?confirm=true`, { method: 'DELETE' });
 
     const msg = await clearedPromise;
     expect(msg.type).toBe('canvas_cleared');