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/migration_registry.txt

@@ -1,35 +0,0 @@
-### AUTOMATICALLY GENERATED DO NOT MODIFY
-###
-### This file is generated by Serverpod when creating a migration, do not modify it
-### manually. If a collision is detected in this file when doing a code merge, resolve
-### the conflict by removing and recreating the conflicting migration.
-
-20260916165410927
-20260930094729362-studysets
-20260930122235111-cards
-20260930145136285-reviews
-20261001064926911-tree-tags-assets
-20261002035419149-file-allowance
-20261002191122122-notes
-20261002211346701-learning-paths
-20261003110855698-pdf-sources
-20261007205440868-card-buried-at
-20261008165051407-pdf-position
-20261008223835206-dictionaries
-20261009123850448-dictionary-kind
-20261009145434785-dictionary-entry-version
-20261009184924904-public-profiles
-20261009195137836-published-studysets
-20261009202105673-explore
-20261009215552429-learner-count
-20261009220324372-based-on
-20261009224606274-feedback
-20261010095547719-assistant-connector
-20261012162411835-studyset-deleted-at
-20261013035552510-dictionary-keys
-20261014153932647-release-colour
-20261015171326465-email-sign-in
-20261015220121195-verified-links
-20261015220626303-discussions
-20261017110130653
-20261019121406578-halkyon-base