Add public availability API and settings panel

Public saatavuus endpoint with API key + CORS protection for
cuitunet.fi website integration. Admin settings tab for API key
management and testing. Includes standalone widget page.

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

.gitignore

@@ -3,6 +3,7 @@ data/users.json
 data/changelog.json
 data/archive.json
 data/leads.json
+data/config.json
 data/reset_tokens.json
 data/login_attempts.json
 data/backups/