feat: add profile image upload with local/S3 storage and Sharp processing

Author: No-bodyq

backend/.gitignore

@@ -54,3 +54,6 @@ pids
 
 # Diagnostic reports (https://nodejs.org/api/report.html)
 report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json
+
+# Local avatar/file uploads
+/uploads