feat(server): tree, tags, review modes, settings and assets (ADR 0004)

The schema of the features planned for the first release, as one migration: every
migration of a synced table locks older app versions out of syncing (ADR 0002, rule 3),
which costs nothing before that release. New card types need no migration; they are the
next version of the revision payload.

- studyset: `parent` (the tree; cycles and orphans show at the top level) and `position`
  among siblings (fractional index; existing studysets at 'a0').
- tag (its name in the payload) and card_tag, one row per assignment, so concurrent
  adds and removes merge.
- review_log.mode: review, reverse (a schedule of its own) or cram (never moves a
  schedule); existing reviews are 'review'.
- person_settings: study goals now, preferences later; random ids, since an id derived
  from the person could be claimed by someone else and break the owner's sync.
- asset: a file's hash, type, size and name; the file goes into Serverpod's private
  cloud storage, served through an owner-checked endpoint (to come).

Migration 20260930211154224-tree-tags-assets only adds: four tables with their space
indexes and foreign keys, three columns with defaults. On devices SQLite rebuilds
studyset and review_log to add foreign keys, which is safe only with foreign keys off,
as Serverpod's runner does; a new test replays every client migration on a database
with a row in each of our synced tables and checks that none is lost (with foreign keys
on, it fails as it should).

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

@@ -8,3 +8,4 @@
 20260930094729362-studysets
 20260930122235111-cards
 20260930145136285-reviews
+20260930211154224-tree-tags-assets