feat: backfill existing decks with default tags

Existing decks created before the tag feature have no deck_tags. Seed the
current default_deck_tags into every deck that has none.

- raw-SQL/local-table migration (no app schemas) so future schema changes
  don't break replay
- idempotent: only decks with zero tags are touched, so re-runs are no-ops

Author: cfbender

priv/repo/migrations/20260708000003_backfill_deck_default_tags.exs

@@ -0,0 +1,41 @@
+defmodule Manavault.Repo.Migrations.BackfillDeckDefaultTags do
+  use Ecto.Migration
+
+  import Ecto.Query
+
+  def up do
+    flush()
+
+    now = DateTime.utc_now() |> DateTime.truncate(:second)
+
+    defaults =
+      repo().all(
+        from(t in "default_deck_tags",
+          order_by: [asc: t.position],
+          select: %{name: t.name, color: t.color, target_count: t.target_count, position: t.position}
+        )
+      )
+
+    tagged_deck_ids =
+      repo().all(from(t in "deck_tags", distinct: true, select: t.deck_id))
+
+    untagged_deck_ids =
+      repo().all(
+        from(d in "decks", where: d.id not in ^tagged_deck_ids, select: d.id)
+      )
+
+    rows =
+      for deck_id <- untagged_deck_ids, default <- defaults do
+        Map.merge(default, %{deck_id: deck_id, inserted_at: now, updated_at: now})
+      end
+
+    if rows != [] do
+      repo().insert_all("deck_tags", rows)
+    end
+  end
+
+  def down do
+    # Data backfill only; cannot distinguish backfilled tags from user-created ones.
+    :ok
+  end
+end