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_hr_notifications.py

@@ -3,16 +3,15 @@ Tests for HR email notifications functionality.
 """
 import pytest
 from unittest.mock import patch, MagicMock
-from app import send_hr_notification
+from app.email_service import send_hr_notification
 
 
 class TestHRNotifications:
     """Test HR notification email functionality."""
 
-    def test_hr_notification_sent_on_submission(self, client, create_test_application, mock_mail):
+    def test_hr_notification_sent_on_submission(self, app, client, create_test_application, mock_mail):
         """Test that HR receives email when application is submitted."""
         # Set HR_EMAIL in config
-        from app import app
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'http://localhost:5000'
 
@@ -59,11 +58,10 @@ class TestHRNotifications:
         assert f'/application/{session_id}/' in hr_email.body
 
     def test_hr_notification_not_sent_when_hr_email_not_configured(
-        self, client, create_test_application, mock_mail, caplog
+        self, app, client, create_test_application, mock_mail, caplog
     ):
         """Test that HR notification is skipped when HR_EMAIL is not configured."""
         # Ensure HR_EMAIL is not set
-        from app import app
         app.config['HR_EMAIL'] = None
 
         # Create a test application
@@ -97,9 +95,8 @@ class TestHRNotifications:
         # Verify warning was logged
         assert 'HR_EMAIL not configured' in caplog.text
 
-    def test_hr_notification_includes_file_count(self, client, create_test_application, mock_mail):
+    def test_hr_notification_includes_file_count(self, app, client, create_test_application, mock_mail):
         """Test that HR notification includes the count of uploaded files."""
-        from app import app
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'http://localhost:5000'
 
@@ -146,9 +143,8 @@ class TestHRNotifications:
         hr_email = mock_mail.sent_messages[0]
         assert 'Anzahl der hochgeladenen Dokumente: 2' in hr_email.body
 
-    def test_hr_notification_with_zero_files(self, client, create_test_application, mock_mail):
+    def test_hr_notification_with_zero_files(self, app, client, create_test_application, mock_mail):
         """Test that HR notification works even when no files are uploaded."""
-        from app import app
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'http://localhost:5000'
 
@@ -184,8 +180,6 @@ class TestHRNotifications:
 
     def test_send_hr_notification_function_directly(self, app):
         """Test the send_hr_notification function directly."""
-        from app import send_hr_notification
-
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'http://localhost:5000'
 
@@ -205,15 +199,13 @@ class TestHRNotifications:
 
         with app.app_context():
             # Mock mail.send to prevent actual sending
-            with patch('app.mail.send') as mock_send:
+            with patch('app.email_service.mail.send') as mock_send:
                 result = send_hr_notification(session_id, app_data)
                 assert result is True
                 assert mock_send.called
 
     def test_send_hr_notification_handles_email_failure(self, app, caplog):
         """Test that send_hr_notification handles email sending failures gracefully."""
-        from app import send_hr_notification
-
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'http://localhost:5000'
 
@@ -231,14 +223,13 @@ class TestHRNotifications:
 
         with app.app_context():
             # Mock mail.send to raise an exception
-            with patch('app.mail.send', side_effect=Exception('SMTP error')):
+            with patch('app.email_service.mail.send', side_effect=Exception('SMTP error')):
                 result = send_hr_notification(session_id, app_data)
                 assert result is False
                 assert 'Failed to send HR notification email' in caplog.text
 
-    def test_hr_notification_url_format(self, client, create_test_application, mock_mail):
+    def test_hr_notification_url_format(self, app, client, create_test_application, mock_mail):
         """Test that the application URL in HR notification is correctly formatted."""
-        from app import app
         app.config['HR_EMAIL'] = 'hr@example.com'
         app.config['APPLICATION_URL_BASE'] = 'https://example.com'