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>

Dockerfile.canvas

@@ -51,6 +51,9 @@ USER nodejs
 
 ENV NODE_ENV=production
 ENV PORT=3000
+# HOST=0.0.0.0 is correct inside Docker: the container binds all interfaces,
+# but external access is gated by the published port mapping in docker-compose.yml.
+# For local dev without Docker, the server defaults to localhost (127.0.0.1).
 ENV HOST=0.0.0.0
 ENV EXCALIDRAW_DB_PATH=/app/data/excalidraw.db