feat(auth): implement OAuth authentication flow

Author: Harxhit

apps/backend/prisma/schema.prisma

@@ -19,7 +19,8 @@ model User {
   displayName String   @map("display_name")
   bio         String?
   pronouns    String?
-  role        Role     @default(USER)
+  role        String?
+  authRole    Role @default(USER)
   company     String?
   avatarUrl   String?  @map("avatar_url")
   accentColor String   @default("#6366f1") @map("accent_color")