♻️ Refactor async tasks implementation.

And migrate the previous unrelated subsystems such that gc and
email sending to the new subsystem.

backend/resources/migrations/0007.tasks.sql

@@ -0,0 +1,17 @@
+CREATE TABLE IF NOT EXISTS tasks (
+  id uuid PRIMARY KEY DEFAULT uuid_generate_v4(),
+
+  created_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+  modified_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+  completed_at timestamptz NULL DEFAULT NULL,
+  scheduled_at timestamptz NOT NULL,
+
+  name text NOT NULL,
+  props bytea NOT NULL,
+
+  retry_num smallint NOT NULL DEFAULT 0,
+  status text NOT NULL DEFAULT 'new'
+);
+
+CREATE INDEX tasks__scheduled_at__idx
+    ON tasks (scheduled_at);