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/validators.py

@@ -0,0 +1,48 @@
+"""
+Validation functions for the Flask job application system.
+"""
+import re
+from flask import current_app
+
+
+def validate_email(email):
+    """Validate email format"""
+    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
+    return re.match(pattern, email) is not None
+
+
+def validate_phone(phone):
+    """Validate international phone format"""
+    # Accepts formats like: +41 79 123 45 67, +41791234567, etc.
+    # Remove spaces and check if it matches the pattern
+    pattern = r'^\+\d{1,3}[\s\d]{1,20}$'
+    if not re.match(pattern, phone):
+        return False
+    # Ensure there are at least some digits after the country code
+    digits_only = re.sub(r'\s', '', phone)
+    return len(digits_only) >= 5  # At least +XX XXX format
+
+
+def validate_year(year):
+    """Validate birth year"""
+    try:
+        year_int = int(year)
+        return (current_app.config['MIN_BIRTH_YEAR'] <= year_int <= current_app.config['MAX_BIRTH_YEAR']
+                and len(str(year)) == 4)
+    except ValueError:
+        return False
+
+
+def validate_zip(zip_code):
+    """Validate ZIP code"""
+    try:
+        zip_int = int(zip_code)
+        return len(str(zip_code)) <= current_app.config['MAX_ZIP_DIGITS']
+    except ValueError:
+        return False
+
+
+def allowed_file(filename):
+    """Check if file extension is allowed"""
+    return '.' in filename and \
+           filename.rsplit('.', 1)[1].lower() in current_app.config['ALLOWED_EXTENSIONS']