feat: implement Story 4.1 - Access Registration Page

Allows potential participants to view exchange details and access
the registration form via unique slug URLs.

Implementation:
- Added ParticipantRegistrationForm with name, email, gift_ideas, and reminder fields
- Created GET /exchange/<slug>/register route
- Built responsive registration template with exchange details
- Exempted participant routes from admin setup requirement
- Comprehensive test coverage for all scenarios

Acceptance Criteria Met:
- Valid slug displays registration form with exchange details
- Invalid slug returns 404
- Form includes all required fields with CSRF protection
- Registration deadline is displayed

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

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

src/routes/participant.py

@@ -1,13 +1,17 @@
 """Participant routes for Sneaky Klaus application."""
 
-from flask import Blueprint
+from flask import Blueprint, abort, render_template
+
+from src.app import db
+from src.forms.participant import ParticipantRegistrationForm
+from src.models.exchange import Exchange
 
 participant_bp = Blueprint("participant", __name__, url_prefix="")
 
 
-@participant_bp.route("/exchange/<slug>/register")
-def register(slug):
-    """Participant registration page (stub for now).
+@participant_bp.route("/exchange/<slug>/register", methods=["GET", "POST"])
+def register(slug: str):
+    """Participant registration page.
 
     Args:
         slug: Exchange registration slug.
@@ -15,4 +19,16 @@ def register(slug):
     Returns:
         Rendered registration page template.
     """
-    return f"Registration page for exchange: {slug}"
+    # Find the exchange by slug
+    exchange = db.session.query(Exchange).filter_by(slug=slug).first()
+    if not exchange:
+        abort(404)
+
+    # Create the registration form
+    form = ParticipantRegistrationForm()
+
+    return render_template(
+        "participant/register.html",
+        exchange=exchange,
+        form=form,
+    )