feat: implement reminder preferences and withdrawal (Stories 6.3, 6.2)

Implement Phase 3 participant self-management features:

Story 6.3 - Reminder Preferences:
- Add ReminderPreferenceForm to participant forms
- Add update_preferences route for preference updates
- Update dashboard template with reminder preference toggle
- Participants can enable/disable reminder emails at any time

Story 6.2 - Withdrawal from Exchange:
- Add can_withdraw utility function for state validation
- Create WithdrawalService to handle withdrawal process
- Add WithdrawForm with explicit confirmation requirement
- Add withdraw route with GET (confirmation) and POST (process)
- Add withdrawal confirmation email template
- Update dashboard to show withdraw link when allowed
- Withdrawal only allowed before registration closes
- Session cleared after withdrawal, user redirected to registration

All acceptance criteria met for both stories.
Test coverage: 90.02% (156 tests passing)
Linting and type checking: passed

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

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

tests/unit/test_withdrawal_service.py

@@ -0,0 +1,67 @@
+"""Unit tests for withdrawal service."""
+
+from datetime import datetime
+from unittest.mock import patch
+
+import pytest
+
+from src.services.withdrawal import WithdrawalError, withdraw_participant
+
+
+def test_withdraw_participant_success(participant_factory, db, app):  # noqa: ARG001
+    """Test successful withdrawal."""
+    with app.app_context():
+        participant = participant_factory()
+
+        with patch("src.services.withdrawal.EmailService") as mock_email_service:
+            withdraw_participant(participant)
+
+            assert participant.withdrawn_at is not None
+            mock_email_service.return_value.send_withdrawal_confirmation.assert_called_once()
+
+
+def test_withdraw_participant_already_withdrawn(participant_factory, app):
+    """Test error when already withdrawn."""
+    with app.app_context():
+        participant = participant_factory(withdrawn_at=datetime.utcnow())
+
+        with pytest.raises(WithdrawalError, match="already withdrawn"):
+            withdraw_participant(participant)
+
+
+def test_withdraw_participant_registration_closed(
+    exchange_factory, participant_factory, app
+):
+    """Test error when registration is closed."""
+    with app.app_context():
+        exchange = exchange_factory(state="registration_closed")
+        participant = participant_factory(exchange=exchange)
+
+        with pytest.raises(WithdrawalError, match="Registration has closed"):
+            withdraw_participant(participant)
+
+
+def test_withdraw_participant_after_matching(
+    exchange_factory, participant_factory, app
+):
+    """Test error when matching has occurred."""
+    with app.app_context():
+        exchange = exchange_factory(state="matched")
+        participant = participant_factory(exchange=exchange)
+
+        with pytest.raises(WithdrawalError, match="Matching has already occurred"):
+            withdraw_participant(participant)
+
+
+def test_withdraw_participant_sets_timestamp(participant_factory, db, app):  # noqa: ARG001
+    """Test that withdrawal sets timestamp correctly."""
+    with app.app_context():
+        participant = participant_factory()
+
+        with patch("src.services.withdrawal.EmailService"):
+            before = datetime.utcnow()
+            withdraw_participant(participant)
+            after = datetime.utcnow()
+
+            assert participant.withdrawn_at is not None
+            assert before <= participant.withdrawn_at <= after