fix: use string budget in email service instead of float

The budget field is stored as a string (e.g., '$25-50'), not a number.
Updated EmailService.send_registration_confirmation() to accept budget
as a string and display it directly in the email.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/participant.py

@@ -136,7 +136,7 @@ def register(slug: str):
                 magic_link_url=magic_link_url,
                 exchange_name=exchange.name,
                 exchange_description=exchange.description,
-                budget_amount=float(exchange.budget.replace("$", "")),
+                budget=exchange.budget,
                 gift_exchange_date=exchange.exchange_date.strftime("%Y-%m-%d"),
             )
         except Exception as e: