fix: Add missing get_db function to database.py

The main.py was trying to import get_db for blockchain initialization
but it was missing from database.py. Added the get_db generator function
that creates and properly closes database sessions.

e-voting-system/backend/database.py

@@ -23,3 +23,12 @@ def init_db():
     """Initialiser la base de données (créer les tables)"""
     from .models import Base
     Base.metadata.create_all(bind=engine)
+
+
+def get_db():
+    """Dépendance pour obtenir une session de base de données"""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()