feat(app): study with FSRS

A studyset's card list gains "Study". The study screen shows the front, then the answer and the four ratings, each labelled with when the card would come back; every rating is recorded with how long answering took, and the queue is worked out anew after each.

Schedules are never stored: scheduleOf replays a card's reviews in time order with FSRS-6 (fsrs v2.0.1, pinned exactly so every device on one app version computes the same). The package's fuzz can only be seeded through an API meant for tests, and due dates don't feed back into a card's state, so the replay runs without fuzz and fuzzedDays spreads the final due date over FSRS's range, picked by a stable FNV-1a fraction of the card's id and review count. studyQueue takes due cards earliest first, then new cards in studyset order, 20 per studyset and local day, and brings a learning card due within 20 minutes early when nothing else is due. Deleting a card deletes its reviews.

Tests: the replay (learning, growing intervals, the same schedule whatever order reviews arrive in, previews matching what a rating yields), the fuzz range, the queue (order, the daily limit counted per local day, learn-ahead), the repository, the session bloc under a controlled clock, the screen, and a sync test for ratings on two devices. AGENTS.md records the review log and scheduling, and how plugin lint rules are ignored.

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

AGENTS.md

@@ -34,7 +34,7 @@ Only when the server cannot be started at all, fall back to the CLI in the serve
 
 Tests need no Docker. `config/test.yaml` sets `database.dataPath`, so Serverpod starts and manages the test database (an embedded PostgreSQL) itself. Just run `dart test` in the server package.
 
-Two tooling traps: `dart fix --apply` moves `flutter_driver` into the app's `dependencies` (as `any`) on every run, so restore it as a dev dependency (`sdk: flutter`) afterwards; and never `dart format` generated code, only the hand-written paths, as `tool/check.sh` does.
+Tooling traps: rules of `leancode_lint`'s analyzer plugin are ignored with its prefix (`// ignore: leancode_lint/missing_equatable_props`); `dart fix --apply` moves `flutter_driver` into the app's `dependencies` (as `any`) on every run, so restore it as a dev dependency (`sdk: flutter`) afterwards; and never `dart format` generated code, only the hand-written paths, as `tool/check.sh` does.
 
 Checklist after doing changes, in this order:
 
@@ -118,6 +118,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 - **Content envelope**: synced content tables carry `keyId` and `nonce` (nullable; `null` = plaintext, the only value, ADR 0002 rule 6) and `payload` (`ByteData`): UTF-8 JSON with a `schemaVersion`, encoded and decoded by one codec per feature (e.g. `studyset_payload.dart`) on top of `core/content/payload.dart`, which also holds the blanked `redactedPayload`. Content fields live inside `payload`: a new field raises the schema version instead of migrating a synced table, which would lock older app versions out of syncing (ADR 0002, rule 3). A payload in a newer schema is skipped with a log, never shown half-read.
 - **Synced data** (`database: sync`, `serverpod_offline_sync`, ADR 0001): every person has their own local SQLite database (`<app support>/people/<authUserId>/sync.sqlite`). Its gateway writes through the `OfflineSyncDatabaseSession`, **names the columns of every update**, and calls `SyncService.requestSync()` after every write; the repository re-reads after its own writes and when a sync merged changes (`remoteChanges`). `SyncService` syncs on triggers (at start, 2 s after the last write, on resume, every 5 min), one sync at a time. **Deleting blanks the payload first** (ADR 0002, rule 2): the engine keeps deleted rows until the space is deleted. Order by the UUID v7 id (ADR 0001, rule 7). Access to synced rows is the engine's: a person syncs their personal space and the spaces granted to them; the endpoint rules above guard everything that isn't synced.
   - **Content people edit is append-only revisions** (ADR 0001 rule 2, ADR 0003; the reference is `CardRevision`): a revision is never updated except to blank it, and `parents` lists the revisions it replaces. A save's parents are all current heads, which is also how a conflict is resolved. `mergeRevisions` (`feature/card/model/card_merge.dart`) merges several heads per side against their newest common ancestor; a side changed differently is a conflict the person settles, never merged silently. Deleting blanks every revision, then deletes.
+  - **Reviews are an append-only log** (`ReviewLog`: card, rating, `reviewedAt` in UTC, `durationMs`). A card's schedule is never stored: `scheduleOf` (`feature/study/model/card_schedule.dart`) replays its reviews in time order with FSRS-6 (`fsrs`, pinned exactly), and only the final due date is fuzzed, from the card's id and review count, so every device computes the same. The study queue (`study_queue.dart`): due cards earliest first, then new cards in studyset order, 20 per studyset and local day. Deleting a card deletes its reviews.
   - **Order is a fractional index** (`fractional_indexing_dart`, rocicorp's algorithm) in its own column, last writer wins. Two devices appending at once can pick the same key, so ties sort by id.
   - **Every synced table indexes `spaceId`** (`indexes: <table>_space_idx: fields: spaceId`, e.g. `studyset.spy.yaml`): deleting a space cascades into every synced table, and Postgres doesn't index foreign keys by itself, so account deletion and pruning would otherwise read whole tables.
   - **The client's local migration** (in `halkyon_learn_client/lib/migrations/`, written by the same `create-migration`) creates the device's SQLite schema from the whole definition, so it also contains the server's own tables. Only the synced tables and the bookkeeping ever hold data on a device.