feat(server): archived and starred studysets (ADR 0004)

studyset gains archivedAt (archived studysets rest in their own section and are left out
of studying, with everything below them) and starredAt (kept at the top of the
library), both nullable. Columns rather than payload fields, for the same reason as the
parent: starring on one device and renaming on another must not overwrite each other.

The branch's migration wasn't applied anywhere yet (tests build fresh databases, and
the branch isn't merged), so it is recreated with the two columns instead of adding a
second one: 20260930213716558-tree-tags-assets replaces 20260930211154224, and differs
from it only by the two ALTER TABLE lines and its version.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/migrations/migration_registry.txt

@@ -8,4 +8,4 @@
 20260930094729362-studysets
 20260930122235111-cards
 20260930145136285-reviews
-20260930211154224-tree-tags-assets
+20260930213716558-tree-tags-assets