improvement: add error codes enum for re-use

backend/src/lib/error-codes/database.ts

@@ -0,0 +1,4 @@
+export enum DatabaseErrorCode {
+  ForeignKeyViolation = "23503",
+  UniqueViolation = "23505"
+}

backend/src/lib/error-codes/index.ts

@@ -0,0 +1 @@
+export * from "./database";