feat: ✨ let database handles creation times

migrations/20241204225143_create_post_table.up.sql

@@ -1,6 +1,6 @@
 -- Add up migration script here
 CREATE TABLE IF NOT EXISTS "post"(
-    creation_time TIMESTAMPTZ PRIMARY KEY UNIQUE NOT NULL,
+    creation_time TIMESTAMPTZ PRIMARY KEY UNIQUE NOT NULL DEFAULT NOW(),
     poster_id BIGSERIAL NOT NULL REFERENCES "user"(id),
     post VARCHAR NOT NULL UNIQUE
 );