feat(studyset): deleted studysets wait 14 days in Recently deleted

Deleting a studyset blanked it with everything inside it once a
four-second Undo was over. It now only marks it (`studyset.deletedAt`, a
column as `archivedAt`): it leaves every list, count and session with
what is inside it and waits in the library's Recently deleted for 14
days, saying how many are left, with Restore and Delete now. Deleting no
longer asks first and offers Undo; Delete now asks. Once the 14 days are
over, whichever of the person's devices looks next (as their data opens,
after every sync) deletes it for good, blanking first as before; the
server can't blank a payload and never writes a person's space (ADR
0007). The connector, publishing and the learner count leave deleted
studysets out too.

The migration adds one nullable column: app versions before it stop
syncing until they update.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/migrations/20261012162411835-studyset-deleted-at/migration.json

@@ -0,0 +1,33 @@
+{
+  "__className__": "serverpod.DatabaseMigration",
+  "actions": [
+    {
+      "__className__": "serverpod.DatabaseMigrationAction",
+      "type": "alterTable",
+      "alterTable": {
+        "__className__": "serverpod.TableMigration",
+        "name": "studyset",
+        "schema": "public",
+        "addColumns": [
+          {
+            "__className__": "serverpod.ColumnDefinition",
+            "name": "deletedAt",
+            "fieldName": "deletedAt",
+            "columnType": 4,
+            "isNullable": true,
+            "dartType": "DateTime?"
+          }
+        ],
+        "deleteColumns": [],
+        "modifyColumns": [],
+        "addIndexes": [],
+        "deleteIndexes": [],
+        "addForeignKeys": [],
+        "deleteForeignKeys": [],
+        "warnings": []
+      }
+    }
+  ],
+  "warnings": [],
+  "migrationApiVersion": 1
+}

halkyon_learn_server/migrations/20261012162411835-studyset-deleted-at/migration.sql

@@ -0,0 +1,49 @@
+BEGIN;
+
+--
+-- ACTION ALTER TABLE
+--
+ALTER TABLE "studyset" ADD COLUMN "deletedAt" timestamp without time zone;
+
+--
+-- MIGRATION VERSION FOR halkyon_learn
+--
+INSERT INTO "serverpod_migrations" ("module", "version", "timestamp")
+    VALUES ('halkyon_learn', '20261012162411835-studyset-deleted-at', now())
+    ON CONFLICT ("module")
+    DO UPDATE SET "version" = '20261012162411835-studyset-deleted-at', "timestamp" = now();
+
+--
+-- MIGRATION VERSION FOR serverpod
+--
+INSERT INTO "serverpod_migrations" ("module", "version", "timestamp")
+    VALUES ('serverpod', '20260824182259319', now())
+    ON CONFLICT ("module")
+    DO UPDATE SET "version" = '20260824182259319', "timestamp" = now();
+
+--
+-- MIGRATION VERSION FOR serverpod_auth_idp
+--
+INSERT INTO "serverpod_migrations" ("module", "version", "timestamp")
+    VALUES ('serverpod_auth_idp', '20260924105404509', now())
+    ON CONFLICT ("module")
+    DO UPDATE SET "version" = '20260924105404509', "timestamp" = now();
+
+--
+-- MIGRATION VERSION FOR serverpod_offline_sync
+--
+INSERT INTO "serverpod_migrations" ("module", "version", "timestamp")
+    VALUES ('serverpod_offline_sync', '20260914143806119', now())
+    ON CONFLICT ("module")
+    DO UPDATE SET "version" = '20260914143806119', "timestamp" = now();
+
+--
+-- MIGRATION VERSION FOR serverpod_auth_core
+--
+INSERT INTO "serverpod_migrations" ("module", "version", "timestamp")
+    VALUES ('serverpod_auth_core', '20260924105232991', now())
+    ON CONFLICT ("module")
+    DO UPDATE SET "version" = '20260924105232991', "timestamp" = now();
+
+
+COMMIT;