add created_at timestamp to task table

Co-authored-by: Vincent Shuali <[email protected]>

src/backend/db/migrations/4_add-created-at-on-tasks.sql

@@ -0,0 +1,7 @@
+-- Each task should have a created_at date so that we can sort by assigned date
+-- NOTE: This relies on the assigned date never changing; if the app allows changes, we will likely want create a dedicated assigned_on date in addition to (or instead of) this column 
+ALTER TABLE task
+ADD COLUMN created_at TIMESTAMPTZ NOT NULL DEFAULT NOW();
+
+-- Add index to allow easy queries of tasks by date
+CREATE INDEX idx_created_at ON task(created_at);