Adds download_at to files, skips files that are impossible to download

migrations/20210729031521_add_downloaded_at_to_files.sql

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+ALTER TABLE "files" ADD COLUMN "downloaded_at" TIMESTAMP(3);
+
+/* Backfill existing columns the created_at timestamps.
+ *
+ * This is approximate since usually the file was downloaded shortly after the record was created.
+ * I mostly only care whether it is null or not null. All existing files need non-null values.
+ */
+UPDATE "files" SET "downloaded_at" = "created_at";