Security hardening: env-based config, CSRF protection, secure code generation, plus presenter management and email communications system

- Move all credentials from hardcoded to .env with python-dotenv (add .env.example template and .gitignore)
- Add CSRF token generation and validation on all state-changing requests
- Replace random.choices with secrets.choice for all secure code generation
- Add session cookie security headers (HttpOnly, Secure, SameSite)
- Add presenter management: assign/remove presenters to breakout sessions, presenter QR scanning
- Add email communications system: templates per event, custom email sending, sent email tracking
- Add staff/organizer profile pages with staff code display
- New DB tables: event_email_templates, sent_emails, user_communications
- New DB columns: staff.reminder_count/reminder_dates, breakout_session_organizers.presenter_code
- Expand English translation strings across all new features

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

.gitignore

@@ -0,0 +1,21 @@
+# Environment
+.env
+
+# Python
+__pycache__/
+*.pyc
+*.pyo
+venv/
+
+# Flask
+instance/
+
+# Uploads
+static/uploads/*
+!static/uploads/.gitkeep
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo