feat: add middleware to enforce JSON Content-Type for API requests

- Introduced `requireJsonContentType` middleware to ensure that all POST, PUT, and PATCH requests have the Content-Type set to application/json.
- This enhancement improves security by preventing CSRF and content-type confusion attacks, ensuring only properly formatted requests are processed.

apps/server/src/middleware/require-json-content-type.ts

@@ -0,0 +1,50 @@
+/**
+ * Middleware to enforce Content-Type: application/json for request bodies
+ *
+ * This security middleware prevents malicious requests by requiring proper
+ * Content-Type headers for all POST, PUT, and PATCH requests.
+ *
+ * Rejecting requests without proper Content-Type helps prevent:
+ * - CSRF attacks via form submissions (which use application/x-www-form-urlencoded)
+ * - Content-type confusion attacks
+ * - Malformed request exploitation
+ */
+
+import type { Request, Response, NextFunction } from 'express';
+
+// HTTP methods that typically include request bodies
+const METHODS_REQUIRING_JSON = ['POST', 'PUT', 'PATCH'];
+
+/**
+ * Middleware that requires Content-Type: application/json for POST/PUT/PATCH requests
+ *
+ * Returns 415 Unsupported Media Type if:
+ * - The request method is POST, PUT, or PATCH
+ * - AND the Content-Type header is missing or not application/json
+ *
+ * Allows requests to pass through if:
+ * - The request method is GET, DELETE, OPTIONS, HEAD, etc.
+ * - OR the Content-Type is properly set to application/json (with optional charset)
+ */
+export function requireJsonContentType(req: Request, res: Response, next: NextFunction): void {
+  // Skip validation for methods that don't require a body
+  if (!METHODS_REQUIRING_JSON.includes(req.method)) {
+    next();
+    return;
+  }
+
+  const contentType = req.headers['content-type'];
+
+  // Check if Content-Type header exists and contains application/json
+  // Allows for charset parameter: "application/json; charset=utf-8"
+  if (!contentType || !contentType.toLowerCase().includes('application/json')) {
+    res.status(415).json({
+      success: false,
+      error: 'Unsupported Media Type',
+      message: 'Content-Type header must be application/json',
+    });
+    return;
+  }
+
+  next();
+}