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>

tests/integration/test_story_4_1_access_registration_page.py

@@ -0,0 +1,152 @@
+"""Tests for Story 4.1: Access Registration Page.
+
+As a potential participant, I want to access the registration page
+via the unique link shared by the admin so that I can sign up for
+the Secret Santa exchange.
+"""
+
+from datetime import datetime, timedelta
+
+from src.models.exchange import Exchange
+
+
+class TestStory41AccessRegistrationPage:
+    """Test suite for Story 4.1: Access Registration Page."""
+
+    def test_get_registration_page_with_valid_slug(self, client, db):
+        """Test registration page with valid slug displays exchange details."""
+        # Create an exchange
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="winter2025",
+            name="Office Secret Santa 2025",
+            description="Annual office gift exchange",
+            budget="$50",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.commit()
+
+        # Access registration page
+        response = client.get("/exchange/winter2025/register")
+
+        # Should return 200 OK
+        assert response.status_code == 200
+
+        # Should display exchange details
+        assert b"Office Secret Santa 2025" in response.data
+        assert b"Annual office gift exchange" in response.data
+        assert b"$50" in response.data
+        # Format the date to match what will be displayed
+        assert future_exchange_date.strftime("%Y-%m-%d").encode() in response.data
+
+        # Should display registration form
+        assert b'name="name"' in response.data
+        assert b'name="email"' in response.data
+        assert b'name="gift_ideas"' in response.data
+        assert b'name="reminder_enabled"' in response.data
+
+    def test_get_registration_page_with_invalid_slug(self, client):
+        """Test accessing registration page with invalid slug returns 404."""
+        response = client.get("/exchange/invalid-slug/register")
+        assert response.status_code == 404
+
+    def test_registration_form_has_csrf_protection(self, client, db):
+        """Test registration form includes POST method (CSRF via Flask-WTF)."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.commit()
+
+        response = client.get("/exchange/test123/register")
+
+        # Should have a POST form (CSRF protection is provided by Flask-WTF)
+        # Note: CSRF tokens are disabled in test mode, so we just verify the form exists
+        assert b'<form method="POST">' in response.data
+        assert b'<button type="submit">Register</button>' in response.data
+
+    def test_registration_page_displays_optional_description(self, client, db):
+        """Test that optional description is displayed if present."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            description="This is a test description",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.commit()
+
+        response = client.get("/exchange/test123/register")
+
+        assert b"This is a test description" in response.data
+
+    def test_registration_page_without_description(self, client, db):
+        """Test that page works when description is not provided."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.commit()
+
+        response = client.get("/exchange/test123/register")
+
+        # Should still work without description
+        assert response.status_code == 200
+        assert b"Test Exchange" in response.data
+
+    def test_registration_page_displays_registration_deadline(self, client, db):
+        """Test that registration deadline is displayed."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.commit()
+
+        response = client.get("/exchange/test123/register")
+
+        assert future_close_date.strftime("%Y-%m-%d").encode() in response.data