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/migrations/20261014153932647-release-colour/migration.json

@@ -0,0 +1,33 @@
+{
+  "__className__": "serverpod.DatabaseMigration",
+  "actions": [
+    {
+      "__className__": "serverpod.DatabaseMigrationAction",
+      "type": "alterTable",
+      "alterTable": {
+        "__className__": "serverpod.TableMigration",
+        "name": "studyset_release",
+        "schema": "public",
+        "addColumns": [
+          {
+            "__className__": "serverpod.ColumnDefinition",
+            "name": "colorName",
+            "fieldName": "colorName",
+            "columnType": 0,
+            "isNullable": true,
+            "dartType": "String?"
+          }
+        ],
+        "deleteColumns": [],
+        "modifyColumns": [],
+        "addIndexes": [],
+        "deleteIndexes": [],
+        "addForeignKeys": [],
+        "deleteForeignKeys": [],
+        "warnings": []
+      }
+    }
+  ],
+  "warnings": [],
+  "migrationApiVersion": 1
+}