feat(store): ListAllUsers for scheduler user enumeration

The in-process scheduler (ADR-018) needs to enumerate every user to run a
discovery pass each. user_identities is the un-RLS'd map; add ListAllUsers as
a plain pool query (no withUser) — the same enumerate-then-act pattern
UserBySubject and the login_events gate query established. Scoping it to a
single user would defeat the point; user_identities carries no RLS by design.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/identity_test.go

@@ -85,6 +85,39 @@ func TestRegisterUserRejectsDuplicateSubject(t *testing.T) {
 	require.Equal(t, first, got)
 }
 
+func TestListAllUsersReturnsEveryIdentity(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	empty, err := s.ListAllUsers(ctx)
+	require.NoError(t, err)
+	require.Empty(t, empty, "no registrations yet → empty slice")
+
+	const subjectC = "dex|carol-789"
+	idA, err := s.RegisterUser(ctx, subjectA, "Alice")
+	require.NoError(t, err)
+	idB, err := s.RegisterUser(ctx, subjectB, "Bob")
+	require.NoError(t, err)
+	idC, err := s.RegisterUser(ctx, subjectC, "Carol")
+	require.NoError(t, err)
+
+	users, err := s.ListAllUsers(ctx)
+	require.NoError(t, err)
+	require.Len(t, users, 3)
+
+	// Order is unspecified; compare as a set of (userID, subject) pairs.
+	got := make(map[string]string, len(users))
+	for _, u := range users {
+		got[u.DexSubject] = u.UserID
+	}
+	require.Equal(t, map[string]string{
+		subjectA: idA,
+		subjectB: idB,
+		subjectC: idC,
+	}, got)
+}
+
 func TestRegisterUserDistinctSubjectsGetDistinctUsers(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)