refactor: split monolithic app.py into modular Flask application with factory pattern

- Created app/ package with 6 modules:
  - __init__.py: Application factory (create_app function)
  - routes.py: All route handlers (473 lines)
  - models.py: Data persistence layer (41 lines)
  - validators.py: Input validation functions (48 lines)
  - email_service.py: Email sending functions (85 lines)
  - utils.py: Utility functions (30 lines)

- Implemented Flask application factory pattern
- Created run.py as minimal entry point
- Updated all test imports to use new module structure
- Fixed template/static folder paths for package structure

- All 102 tests passing
- Improved maintainability with clear separation of concerns
- Follows Flask best practices for scalable applications

tests/conftest.py

@@ -8,7 +8,7 @@ from io import BytesIO
 from datetime import datetime
 import pytest
 import yaml
-from app import app as flask_app
+from app import create_app
 from flask_mail import Mail
 
 
@@ -18,6 +18,9 @@ def app():
     # Create a temporary directory for test applications
     temp_dir = tempfile.mkdtemp()
 
+    # Create test Flask app using factory
+    flask_app = create_app()
+
     # Configure app for testing
     flask_app.config['TESTING'] = True
     flask_app.config['SECRET_KEY'] = 'test-secret-key'
@@ -133,7 +136,9 @@ def mock_mail(app, monkeypatch):
             self.sent_messages.append(message)
 
     mock = MockMail()
-    monkeypatch.setattr('app.mail', mock)
+    # Patch the mail instance in the app package (used by email_service)
+    import app.email_service
+    monkeypatch.setattr(app.email_service, 'mail', mock)
     return mock
 
 
@@ -142,7 +147,7 @@ def create_test_application(app, temp_applications_dir):
     """Factory fixture to create test application data."""
     def _create(session_id='test-123', **kwargs):
         """Create a test application with the given session ID and data."""
-        from app import save_application_data
+        from app.models import save_application_data
 
         data = {
             'session_id': session_id,