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/test_integration.py

@@ -3,7 +3,7 @@ Integration tests for the complete application workflow.
 """
 import pytest
 from unittest.mock import patch
-from app import load_application_data
+from app.models import load_application_data
 
 
 @pytest.mark.integration
@@ -13,7 +13,7 @@ class TestCompleteWorkflow:
     def test_complete_application_workflow(self, client, app, sample_pdf_file):
         """Test the complete workflow from email to confirmation."""
         # Step 1: Submit email
-        with patch('app.mail.send'):
+        with patch('app.email_service.mail.send'):
             response = client.post('/apply/submit-email', data={
                 'email': 'integration@test.com',
                 'job_name': 'Integration Test Position'
@@ -22,7 +22,7 @@ class TestCompleteWorkflow:
             assert response.status_code == 200
 
         # For testing, create a known session
-        from app import save_application_data
+        from app.models import save_application_data
         session_id = 'integration-test-123'
         save_application_data(session_id, {
             'session_id': session_id,
@@ -130,7 +130,7 @@ class TestCompleteWorkflow:
 
     def test_multiple_concurrent_applications(self, client, app):
         """Test handling multiple concurrent applications with different sessions."""
-        from app import save_application_data
+        from app.models import save_application_data
 
         # Create multiple applications
         sessions = []
@@ -182,7 +182,7 @@ class TestWorkflowEdgeCases:
 
     def test_skip_optional_fields(self, client, app):
         """Test completing workflow without filling optional fields."""
-        from app import save_application_data
+        from app.models import save_application_data
 
         session_id = 'optional-test'
         save_application_data(session_id, {
@@ -226,7 +226,7 @@ class TestWorkflowEdgeCases:
 
     def test_workflow_with_unicode_data(self, client, app):
         """Test workflow with German special characters."""
-        from app import save_application_data
+        from app.models import save_application_data
 
         session_id = 'unicode-test'
         save_application_data(session_id, {
@@ -265,7 +265,7 @@ class TestDataIntegrity:
 
     def test_data_not_lost_between_pages(self, client, app):
         """Test that data persists correctly when navigating between pages."""
-        from app import save_application_data
+        from app.models import save_application_data
 
         session_id = 'persistence-test'
         save_application_data(session_id, {