fix: utiliser npm ci et ajouter vérification express dans Dockerfile

Dockerfile

@@ -5,8 +5,11 @@ WORKDIR /app
 # Copy package files
 COPY backend/package*.json ./
 
-# Install dependencies cleanly
-RUN npm install --production
+# Install dependencies with npm ci for reproducibility
+RUN npm ci --only=production || npm install --production
+
+# Verify critical dependencies are installed
+RUN test -d node_modules/express || (echo "ERROR: express not installed" && exit 1)
 
 # Copy application code
 COPY backend/src ./src