Term status model: single source of truth + Anki/FSRS-aligned scheduling

[HugoFara]

Status: Proposed — deferred until after the next release (the FSRS part is an architectural change worth landing on its own).

Proposal doc in-repo: docs-src/developer/term-status-fsrs.md.

Problem

The word-status model (1-5 learning, 98 ignored, 99 well-known) is the spine of both the reading UI (word colouring) and the review system, but it is modelled ad-hoc:

• Duplicated: the literal [1,2,3,4,5,98,99] and checks like $status === 5 || $status === 99 recur across 11+ PHP files; label/colour/order/CSS tables are re-defined in ~6 TS files. A TermStatus value object exists (src/Modules/Vocabulary/Domain/ValueObject/TermStatus.php) but isn't the single source of truth.
• Scheduling is a hand-tuned Leitner curve: TermStatusService::SCORE_FORMULA_TODAY/TOMORROW is a per-status linear decay stored in WoTodayScore/WoTomorrowScore, shuffled by WoRandom; a review just nudges status ±1. No memory model, no per-term difficulty, no retention target, no review history.

One integer conflates two concerns: how familiar a word is (reading view) and when it should next be reviewed (scheduling).

Phase 1 — status as a single source of truth (independent, low-risk)

Promote TermStatus to authoritative (value, label, abbr, cssClass, colour, order, isKnown()/isIgnored()/isLearning()); fold in TermStatusService/StatusHelper; replace the ~11 PHP literals; expose once to the frontend (bootstrap or GET /api/v1/settings/status-definitions) and collapse the ~6 duplicated TS tables into one store.

Phase 2 — FSRS-aligned scheduling

Split display familiarity from memory state:

Concern	Today	Proposed
Display familiarity (colours)	WoStatus 1–5	keep 1–5, derived from stability
Scheduling	per-status decay score	FSRS memory state per term

FSRS models each term with Stability (S), Difficulty (D), Retrievability (R) (power forgetting curve; due when R hits target retention, default 0.9). Reviews become 4 grades (Again/Hard/Good/Easy), each updating S/D and the next due date.

Changes: schedule columns / term_schedule + review_log tables (retire WoTodayScore/WoTomorrowScore/WoRandom + formulas); a swappable Scheduler service (FSRS impl, interface-backed); SubmitAnswer calls the scheduler; 4-grade review UI; derive 1–5 colours by bucketing S; 98/99 stay manual flags (≈ suspended/known). Existing terms seed FSRS state from WoStatus + WoStatusChanged.

Decisions to make

1. Display status derived from stability (recommended, with manual override) vs. fully manual/orthogonal.
2. 4-grade vs. a 2-button (Again/Good) mode for existing users.
3. Vendor a maintained PHP FSRS port vs. hand-port open-spaced-repetition (licence check before vendoring).

See the proposal doc for migration, scope, and verification detail.