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_application_view.py

@@ -174,7 +174,7 @@ class TestFileDownload:
         session_id = 'test-download-success'
 
         # Create application directory and attachment
-        from app import get_attachments_path
+        from app.models import get_attachments_path
         attachments_path = get_attachments_path(session_id)
         Path(attachments_path).mkdir(parents=True, exist_ok=True)
 
@@ -215,7 +215,7 @@ class TestFileDownload:
         session_id = 'test-download-unauthorized'
 
         # Create application directory and attachment
-        from app import get_attachments_path
+        from app.models import get_attachments_path
         attachments_path = get_attachments_path(session_id)
         Path(attachments_path).mkdir(parents=True, exist_ok=True)
 
@@ -273,7 +273,7 @@ class TestFileDownload:
         session_id = 'test-download-multiple'
 
         # Create application directory
-        from app import get_attachments_path
+        from app.models import get_attachments_path
         attachments_path = get_attachments_path(session_id)
         Path(attachments_path).mkdir(parents=True, exist_ok=True)
 
@@ -318,7 +318,7 @@ class TestFileDownload:
         session_id = 'test-download-special-chars'
 
         # Create application directory
-        from app import get_attachments_path
+        from app.models import get_attachments_path
         attachments_path = get_attachments_path(session_id)
         Path(attachments_path).mkdir(parents=True, exist_ok=True)