maintenance: break maintenance tasks into individual jobs.

Break the hourly/daily/weekly/monthly maintenance tasks down into
individual delayed jobs. This way if one task fails, it won't prevent
other tasks from running. Also, jobs can be run in parallel, and can be
individually retried if they fail.

app/jobs/vacuum_database_job.rb

@@ -0,0 +1,8 @@
+# A job that runs daily to vacuum the database. Spawned by {DanbooruMaintenance}.
+class VacuumDatabaseJob < ApplicationJob
+  def perform
+    # We can't perform vacuum inside a transaction. This happens during tests.
+    return if ApplicationRecord.connection.transaction_open?
+    ApplicationRecord.connection.execute("vacuum analyze")
+  end
+end