feat(publish): a release keeps its colour, and a description for Explore

Publishing takes a description for Explore (rich text, as its JSON),
which Explore shows in place of the studyset's own: a studyset may
introduce itself to strangers otherwise than in the library, where its
author may leave it empty. Without one, as from older apps, the
studyset's description as before; one that isn't rich text, or over
100,000 characters, counts as none. A release also keeps the studyset's
colour, which Explore gives out with it; releases before have none. The
publication's releases say their description, for the next publish to
start from.

Migration release-colour adds a nullable column:
ALTER TABLE "studyset_release" ADD COLUMN "colorName" text;
the server before it reads and writes the table as ever.

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/test/integration/publish/publish_test.dart

@@ -162,6 +162,7 @@ void main() {
                 'license': ?license,
                 'licenseHolder': 'Alice',
                 'study': {'newPerDay': 10},
+                'color': 'cobalt',
                 'published': ?copyOf,
                 'examPlans': [
                   {
@@ -528,6 +529,47 @@ void main() {
         );
       });
 
+      test("a release keeps the studyset's colour, and what its author wrote "
+          'for Explore as its description; what is no rich text counts as '
+          'nothing written', () async {
+        await profile(alice, 'alice');
+        final content = await japanese(alice);
+        final about = jsonEncode({
+          'formatVersion': 1,
+          'editor': {
+            'document': {'type': 'page'},
+          },
+        });
+
+        final published = await endpoints.publish.publish(
+          signedIn(alice),
+          content.root,
+          'japanese',
+          '',
+          description: about,
+        );
+
+        expect(published.releases.single.description, about);
+        final kept = await StudysetRelease.db.find(session);
+        expect(kept.single.colorName, 'cobalt');
+        expect(kept.single.description, about);
+
+        await revise(
+          alice,
+          content.word,
+          await revisionsOf(content.word),
+          '日本語',
+        );
+        final second = await endpoints.publish.publish(
+          signedIn(alice),
+          content.root,
+          'japanese',
+          '',
+          description: 'not rich text',
+        );
+        expect(second.releases.first.description, isNull);
+      });
+
       test(
         'a version git has but the database lost is never given again',
         () async {