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

app/email_service.py

@@ -0,0 +1,85 @@
+"""
+Email service functions for the Flask job application system.
+"""
+from datetime import datetime
+from flask import url_for, current_app
+from flask_mail import Message
+from app import mail
+
+
+def send_resume_email(email, session_id, job_name):
+    """Send email with resume link"""
+    resume_link = url_for('resume_application', session_id=session_id, _external=True)
+
+    subject = f"Ihre Bewerbung bei {current_app.config['COMPANY_NAME']} - Link zum Fortsetzen"
+    body = f"""Guten Tag,
+
+vielen Dank für Ihr Interesse an der Position "{job_name}" bei {current_app.config['COMPANY_NAME']}.
+
+Sie können Ihre Bewerbung jederzeit über den folgenden Link fortsetzen:
+{resume_link}
+
+Dieser Link bleibt gültig und Sie können Ihre Bewerbung jederzeit bearbeiten.
+
+Mit freundlichen Grüßen
+{current_app.config['COMPANY_NAME']}
+"""
+
+    try:
+        msg = Message(subject=subject, recipients=[email], body=body)
+        mail.send(msg)
+        return True
+    except Exception as e:
+        current_app.logger.error(f"Failed to send email: {e}")
+        return False
+
+
+def send_hr_notification(session_id, app_data):
+    """Send HR notification email when application is submitted"""
+    if not current_app.config.get('HR_EMAIL'):
+        current_app.logger.warning("HR_EMAIL not configured, skipping HR notification")
+        return False
+
+    # Build application URL
+    application_url = f"{current_app.config['APPLICATION_URL_BASE']}/application/{session_id}/"
+
+    # Get personal info
+    personal_info = app_data.get('personal_info', {})
+    firstname = personal_info.get('firstname', 'N/A')
+    name = personal_info.get('name', 'N/A')
+    email = app_data.get('email', 'N/A')
+    job_name = app_data.get('job_name', 'N/A')
+    file_count = len(app_data.get('uploaded_files', []))
+
+    # Format timestamp
+    submitted_at = app_data.get('submitted_at', datetime.now().isoformat())
+    try:
+        timestamp = datetime.fromisoformat(submitted_at).strftime('%d.%m.%Y %H:%M')
+    except (ValueError, TypeError):
+        timestamp = submitted_at
+
+    subject = f"Neue Bewerbung eingegangen: {job_name}"
+    body = f"""Sehr geehrtes HR-Team,
+
+es ist eine neue Bewerbung eingegangen:
+
+Position: {job_name}
+Name: {firstname} {name}
+E-Mail: {email}
+Eingereicht am: {timestamp}
+Anzahl der hochgeladenen Dokumente: {file_count}
+
+Sie können die vollständige Bewerbung hier einsehen:
+{application_url}
+
+Mit freundlichen Grüßen,
+Ihr Bewerbungssystem
+"""
+
+    try:
+        msg = Message(subject=subject, recipients=[current_app.config['HR_EMAIL']], body=body)
+        mail.send(msg)
+        return True
+    except Exception as e:
+        current_app.logger.error(f"Failed to send HR notification email: {e}")
+        return False