feat: add vocabulary review planning

supabase/migrations/202606210014_vocabulary_review_plan.sql

@@ -0,0 +1,31 @@
+alter table public.user_word_progress
+  add column if not exists review_count integer not null default 0,
+  add column if not exists correct_streak integer not null default 0,
+  add column if not exists ease_factor numeric(4,2) not null default 2.50,
+  add column if not exists last_result text check (last_result in ('correct', 'wrong', 'known', 'unknown')),
+  add column if not exists due_level text not null default 'new' check (due_level in ('new', 'again', 'soon', 'later', 'mastered')),
+  add column if not exists metadata jsonb not null default '{}'::jsonb;
+
+do $$
+begin
+  if not exists (select 1 from pg_constraint where conname = 'user_word_progress_review_count_check') then
+    alter table public.user_word_progress
+      add constraint user_word_progress_review_count_check check (review_count >= 0);
+  end if;
+
+  if not exists (select 1 from pg_constraint where conname = 'user_word_progress_correct_streak_check') then
+    alter table public.user_word_progress
+      add constraint user_word_progress_correct_streak_check check (correct_streak >= 0);
+  end if;
+
+  if not exists (select 1 from pg_constraint where conname = 'user_word_progress_ease_factor_check') then
+    alter table public.user_word_progress
+      add constraint user_word_progress_ease_factor_check check (ease_factor >= 1.30 and ease_factor <= 3.00);
+  end if;
+end $$;
+
+create index if not exists idx_user_word_progress_due
+  on public.user_word_progress(tenant_id, user_id, next_review_date, status, due_level);
+
+create index if not exists idx_vocabulary_words_unit_order
+  on public.vocabulary_words(tenant_id, unit_id, is_active, sort_order, created_at);