feat: ✨ let database handles creation times

migrations/20241204225151_create_comment_table.up.sql

@@ -1,7 +1,7 @@
 -- Add up migration script here
 CREATE TABLE IF NOT EXISTS "comment"(
     post_creation_time TIMESTAMPTZ NOT NULL REFERENCES "post"(creation_time),
-    creation_time TIMESTAMPTZ PRIMARY KEY NOT NULL UNIQUE,
+    creation_time TIMESTAMPTZ PRIMARY KEY NOT NULL UNIQUE DEFAULT NOW(),
     commenter_id BIGSERIAL NOT NULL REFERENCES "user"(id),
     comment VARCHAR NOT NULL
 );