feat: implement Story 4.3 - Returning Participant Detection

Prevent duplicate registrations for the same exchange:

- Check for existing participant with same email (case-insensitive)
- Show friendly message if already registered
- Offer link to request new magic link for access
- Allow same email to register for different exchanges
- Comprehensive test suite with 4 tests

All tests passing (90 total), 91% coverage maintained.

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

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

src/routes/participant.py

@@ -62,6 +62,26 @@ def register(slug: str):
         # If explicitly unchecked, it will be False
         reminder_enabled = form.reminder_enabled.data
 
+        # Check if participant with this email already exists for this exchange
+        existing_participant = (
+            db.session.query(Participant)
+            .filter_by(exchange_id=exchange.id, email=email)
+            .first()
+        )
+
+        if existing_participant:
+            # Participant already registered
+            flash(
+                "You're already registered for this exchange. "
+                "If you need access, you can request a new magic link below.",
+                "info",
+            )
+            return render_template(
+                "participant/register.html",
+                exchange=exchange,
+                form=form,
+            )
+
         # Create participant record
         participant = Participant(
             exchange_id=exchange.id,