Better sqlite timestamp default and pruning query

The `CURRENT_TIMESTAMP` type lacks a trailing `z`, which causes it to be
compared incorrectly against UTC datetimes. Any jobs inserted without a
`scheduled_at`, where the default is used, could be returned in queries
that compare against a `DateTime`.

This prevents the issue in the future two ways:

1. Switch the default `inserted_at/scheduled_at` timestamp to a format
   that can be queried properly.
2. Change the pruning check to use `completed_at` rather than
   `scheduled_at` for existing databases. This is a more accurate query
   that was avoided before because it didn't match the `Basic` engine.

Closes #1391

Author: sorentwo

lib/oban/engines/lite.ex

@@ -146,7 +146,7 @@ defmodule Oban.Engines.Lite do
     select_query =
       queryable
       |> select([j], map(j, [:id, :queue, :state]))
-      |> where([j], j.state == "completed" and j.scheduled_at < ^time)
+      |> where([j], j.state == "completed" and j.completed_at < ^time)
       |> or_where([j], j.state == "cancelled" and j.cancelled_at < ^time)
       |> or_where([j], j.state == "discarded" and j.discarded_at < ^time)
       |> limit(^limit)

lib/oban/migrations/sqlite.ex

@@ -20,8 +20,13 @@ defmodule Oban.Migrations.SQLite do
       add :max_attempts, :integer, null: false, default: 20
       add :priority, :integer, null: false, default: 0
 
-      add :inserted_at, :utc_datetime_usec, null: false, default: fragment("CURRENT_TIMESTAMP")
-      add :scheduled_at, :utc_datetime_usec, null: false, default: fragment("CURRENT_TIMESTAMP")
+      add :inserted_at, :utc_datetime_usec,
+        null: false,
+        default: fragment("(strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))")
+
+      add :scheduled_at, :utc_datetime_usec,
+        null: false,
+        default: fragment("(strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))")
 
       add :attempted_at, :utc_datetime_usec
       add :attempted_by, :json, null: false, default: []