chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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/20260916165410927/definition_project.json

@@ -1,24 +0,0 @@
-{
-  "__className__": "serverpod.DatabaseDefinition",
-  "schemaVersion": 2,
-  "moduleName": "halkyon_learn",
-  "tables": [],
-  "installedModules": [
-    {
-      "__className__": "serverpod.DatabaseMigrationVersion",
-      "module": "serverpod",
-      "version": "20260824182259319"
-    },
-    {
-      "__className__": "serverpod.DatabaseMigrationVersion",
-      "module": "serverpod_auth_idp",
-      "version": "20260910193913364-string-rate-limit-keys"
-    },
-    {
-      "__className__": "serverpod.DatabaseMigrationVersion",
-      "module": "serverpod_auth_core",
-      "version": "20260824182354731"
-    }
-  ],
-  "migrationApiVersion": 1
-}