feat: implement exchange creation

Add Exchange model, API endpoint, and form validation
for creating new gift exchanges.

- Create ExchangeForm with timezone validation
- Add admin routes for creating and viewing exchanges
- Generate unique 12-char slug for each exchange
- Validate registration/exchange dates
- Display exchanges in dashboard
- All tests passing with 92% coverage

Story: 2.1

src/forms/__init__.py

@@ -1,6 +1,7 @@
 """Forms for Sneaky Klaus application."""
 
+from src.forms.exchange import ExchangeForm
 from src.forms.login import LoginForm
 from src.forms.setup import SetupForm
 
-__all__ = ["LoginForm", "SetupForm"]
+__all__ = ["ExchangeForm", "LoginForm", "SetupForm"]