feat: add HTMX web UI with login/register forms

- Home page showing README and stats
- Login/register forms with HTMX
- Registry and activities pages
- Cookie-based auth for web UI
- JWT secret from Docker secrets (/run/secrets/jwt_secret)
- Updated README with secret generation instructions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

requirements.txt

@@ -4,3 +4,5 @@ requests>=2.31.0
 cryptography>=42.0.0
 passlib[bcrypt]>=1.7.4
 python-jose[cryptography]>=3.3.0
+markdown>=3.5.0
+python-multipart>=0.0.6