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>

.env.example

@@ -0,0 +1,28 @@
+# NetEvents Environment Configuration
+# Copy this file to .env and fill in your values
+
+# Flask
+SECRET_KEY=change-me-to-a-random-string
+FLASK_DEBUG=false
+PORT=5002
+
+# Database
+DB_HOST=localhost
+DB_PORT=3306
+DB_USER=root
+DB_PASSWORD=
+DB_NAME=netevent
+DB_APP_USER=netevent_app
+DB_APP_PASSWORD=
+
+# Email (Brevo/Sendinblue)
+MAIL_SERVER=smtp-relay.brevo.com
+MAIL_PORT=587
+MAIL_USE_TLS=true
+MAIL_USERNAME=
+MAIL_PASSWORD=
+MAIL_DEFAULT_SENDER=
+
+# reCAPTCHA
+RECAPTCHA_SITE_KEY=
+RECAPTCHA_SECRET_KEY=