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/app.py

@@ -57,10 +57,12 @@ def create_app(config_name: str | None = None) -> Flask:
 
     # Register blueprints
     from src.routes.admin import admin_bp
+    from src.routes.participant import participant_bp
     from src.routes.setup import setup_bp
 
     app.register_blueprint(setup_bp)
     app.register_blueprint(admin_bp)
+    app.register_blueprint(participant_bp)
 
     # Register error handlers
     register_error_handlers(app)