feat(server): the review log, synced and append-only

ReviewLog keeps every rating of a card: the card, the rating (FSRS's 1 again to 4 easy), when (UTC) and how long answering took, as decided. It is never changed; each device derives a card's schedule by replaying its reviews, so reviews from different devices never conflict. No device id: the sync engine knows each row's device.

The migration (by CLI) only adds: the table, an index on spaceId for space deletion, one for a card's reviews in time order, and the foreign keys (from the space with ON DELETE CASCADE; from the card with ON DELETE CASCADE, DEFERRABLE INITIALLY DEFERRED).

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/lib/src/review/review_log.spy.yaml

@@ -0,0 +1,25 @@
+### One rating of a card, never changed once written: a card's schedule isn't
+### stored but derived on each device by replaying its reviews in time order,
+### so reviews from different devices never conflict (decided 2026-09-30).
+### Synced; the review goes with its card.
+class: ReviewLog
+table: review_log
+database: sync
+fields:
+  ### The card reviewed.
+  card: Card?, relation(onDelete=Cascade, deferred)
+  ### How well it was recalled, FSRS's ratings: 1 again, 2 hard, 3 good,
+  ### 4 easy.
+  rating: int
+  ### When, in UTC.
+  reviewedAt: DateTime
+  ### How long answering took, from showing the card to rating it, in
+  ### milliseconds.
+  durationMs: int
+indexes:
+  # Deleting a space finds its rows through this index (see studyset.spy.yaml).
+  review_log_space_idx:
+    fields: spaceId
+  # A card's reviews, in time order.
+  review_log_card_idx:
+    fields: cardId, reviewedAt