refactor: restructure codebase into modular architecture

- Split monolithic app.py (400+ lines) into organized modules
- Extract models, forms, and commands into separate files
- Implement Flask blueprints for route organization
- Maintain full functionality with cleaner architecture
- Update all templates to use new blueprint URLs
- Enhance README with technical documentation

Structure:
├── app.py (50 lines) - Flask app factory
├── models.py (62 lines) - SQLAlchemy models
├── forms.py (43 lines) - WTForms definitions
├── commands.py (74 lines) - CLI commands
└── routes/ - Blueprint modules for each feature

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

templates/base.html

@@ -19,7 +19,7 @@
                     <a href="{{ url_for('index') }}" class="hover:text-blue-200 transition-colors">Accueil</a>
                     <a href="{{ url_for('classes') }}" class="hover:text-blue-200 transition-colors">Classes</a>
                     <a href="{{ url_for('students') }}" class="hover:text-blue-200 transition-colors">Élèves</a>
-                    <a href="{{ url_for('assessments') }}" class="hover:text-blue-200 transition-colors">Évaluations</a>
+                    <a href="{{ url_for('assessments.list') }}" class="hover:text-blue-200 transition-colors">Évaluations</a>
                 </div>
             </div>
         </div>