chore: ruff format after mypy fixes

fix: bind to 0.0.0.0 for Docker, add __main__ guard, fix libgdk-pixbuf dep

Security hardening: SSRF validation, XSS escapes, rate limiting, proxy default 127.0.0.1, path traversal, webhook validation, security headers

Add Steganographic Detection Engine: 4 new detectors (unicode_adv, stego_markers, entropy, image_stego) + tests + fixtures

Initial release: Palisade Scanner v0.1.0 — web content security for AI agents