feat: change profile pictures

I keep forgetting to do things. The edit mii api route has been using
the public folder as an uploads directory... whoops...

prisma/migrations/20250509211916_profile_pictures/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN     "imageUpdatedAt" TIMESTAMP(3);

prisma/schema.prisma

@@ -19,6 +19,7 @@ model User {
   updatedAt DateTime @updatedAt
 
   usernameUpdatedAt DateTime?
+  imageUpdatedAt    DateTime?
 
   accounts Account[]
   sessions Session[]