fix(merge): carry moves/hints and per-variant best moves on account merge

The account merge predated the recent statistics additions and silently dropped
them:
- account_stats.moves and hints_used were not summed into the primary (the
  secondary's row is read, then deleted), so the secondary's lifetime moves and
  hints were lost.
- account_best_move was not handled at all; the secondary is only tombstoned
  (not deleted), so its per-variant best moves lingered on the dead account and
  never reached the merged statistics screen.

mergeStats now also sums moves + hints_used; a new mergeBestMoves folds the
secondary's best moves into the primary keeping the higher-scoring play per
variant (the rule the per-game upsert uses), then deletes the secondary's rows.
Profile fields were already correct (hint wallet summed, paid_account ORed; no
new accounts columns since).

Tests: TestAccountMergeCore extended — moves/hints summed, best moves merged
(higher score per variant kept), secondary rows cleaned up. Docs: ARCHITECTURE §4.

backend/internal/accountmerge/merge.go

@@ -1,5 +1,6 @@
 // Package accountmerge retires a secondary account into a primary one in a single
-// transaction: it sums statistics and the hint wallet, ORs the paid flag, repoints
+// transaction: it sums statistics (merging the per-variant best moves), sums the hint
+// wallet, ORs the paid flag, repoints
 // the secondary's identities, transfers its games/chat/complaints/invitations,
 // de-duplicates friends and blocks, and leaves the secondary as an audit tombstone
 // (accounts.merged_into). It is the data core of account linking & merge
@@ -68,6 +69,9 @@ func (m *Merger) Merge(ctx context.Context, primary, secondary uuid.UUID) error
 		if err := mergeStats(ctx, tx, primary, secondary, now); err != nil {
 			return err
 		}
+		if err := mergeBestMoves(ctx, tx, primary, secondary, now); err != nil {
+			return err
+		}
 		if err := mergeAccountFields(ctx, tx, primary, secondary, now); err != nil {
 			return err
 		}
@@ -147,8 +151,8 @@ func activeGameIDs(ctx context.Context, tx *sql.Tx, accountID uuid.UUID) ([]uuid
 	return out, nil
 }
 
-// mergeStats folds secondary's lifetime statistics into primary (wins/losses/draws
-// summed, max points kept) and deletes the secondary row.
+// mergeStats folds secondary's lifetime statistics into primary (wins/losses/draws and
+// the moves/hints-used counters summed, max points kept) and deletes the secondary row.
 func mergeStats(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, now time.Time) error {
 	var sec model.AccountStats
 	err := postgres.SELECT(table.AccountStats.AllColumns).
@@ -178,13 +182,16 @@ func mergeStats(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, n
 
 	upd := table.AccountStats.UPDATE(
 		table.AccountStats.Wins, table.AccountStats.Losses, table.AccountStats.Draws,
-		table.AccountStats.MaxGamePoints, table.AccountStats.MaxWordPoints, table.AccountStats.UpdatedAt,
+		table.AccountStats.MaxGamePoints, table.AccountStats.MaxWordPoints,
+		table.AccountStats.Moves, table.AccountStats.HintsUsed, table.AccountStats.UpdatedAt,
 	).SET(
 		postgres.Int(int64(pri.Wins+sec.Wins)),
 		postgres.Int(int64(pri.Losses+sec.Losses)),
 		postgres.Int(int64(pri.Draws+sec.Draws)),
 		postgres.Int(int64(max(pri.MaxGamePoints, sec.MaxGamePoints))),
 		postgres.Int(int64(max(pri.MaxWordPoints, sec.MaxWordPoints))),
+		postgres.Int(int64(pri.Moves+sec.Moves)),
+		postgres.Int(int64(pri.HintsUsed+sec.HintsUsed)),
 		postgres.TimestampzT(now),
 	).WHERE(table.AccountStats.AccountID.EQ(postgres.UUID(primary)))
 	if _, err := upd.ExecContext(ctx, tx); err != nil {
@@ -198,6 +205,41 @@ func mergeStats(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, n
 	return nil
 }
 
+// mergeBestMoves folds secondary's per-variant best moves into primary, keeping the
+// higher-scoring play per variant (the same rule the per-game upsert uses), then deletes
+// the secondary's rows — the secondary is only tombstoned, not removed, so without this
+// they would linger on a dead account and never reach the merged statistics screen.
+func mergeBestMoves(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, now time.Time) error {
+	var srows []model.AccountBestMove
+	err := postgres.SELECT(table.AccountBestMove.AllColumns).
+		FROM(table.AccountBestMove).
+		WHERE(table.AccountBestMove.AccountID.EQ(postgres.UUID(secondary))).
+		QueryContext(ctx, tx, &srows)
+	if err != nil && !errors.Is(err, qrm.ErrNoRows) {
+		return fmt.Errorf("accountmerge: load secondary best moves: %w", err)
+	}
+	for _, s := range srows {
+		ins := table.AccountBestMove.
+			INSERT(table.AccountBestMove.AccountID, table.AccountBestMove.Variant,
+				table.AccountBestMove.Score, table.AccountBestMove.Tiles, table.AccountBestMove.UpdatedAt).
+			VALUES(primary, s.Variant, s.Score, s.Tiles, postgres.TimestampzT(now)).
+			ON_CONFLICT(table.AccountBestMove.AccountID, table.AccountBestMove.Variant).
+			DO_UPDATE(postgres.SET(
+				table.AccountBestMove.Score.SET(table.AccountBestMove.EXCLUDED.Score),
+				table.AccountBestMove.Tiles.SET(table.AccountBestMove.EXCLUDED.Tiles),
+				table.AccountBestMove.UpdatedAt.SET(table.AccountBestMove.EXCLUDED.UpdatedAt),
+			).WHERE(table.AccountBestMove.EXCLUDED.Score.GT(table.AccountBestMove.Score)))
+		if _, err := ins.ExecContext(ctx, tx); err != nil {
+			return fmt.Errorf("accountmerge: merge best move %s: %w", s.Variant, err)
+		}
+	}
+	del := table.AccountBestMove.DELETE().WHERE(table.AccountBestMove.AccountID.EQ(postgres.UUID(secondary)))
+	if _, err := del.ExecContext(ctx, tx); err != nil {
+		return fmt.Errorf("accountmerge: delete secondary best moves: %w", err)
+	}
+	return nil
+}
+
 // mergeAccountFields adds secondary's hint wallet to primary and ORs the paid flag;
 // all other profile fields stay the primary's.
 func mergeAccountFields(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, now time.Time) error {

backend/internal/inttest/merge_test.go

@@ -30,6 +30,34 @@ func setStats(t *testing.T, id uuid.UUID, w, l, d, mg, mw int) {
 	}
 }
 
+func setStatsCounts(t *testing.T, id uuid.UUID, moves, hints int) {
+	t.Helper()
+	if _, err := testDB.ExecContext(context.Background(),
+		`UPDATE backend.account_stats SET moves=$2, hints_used=$3 WHERE account_id=$1`, id, moves, hints); err != nil {
+		t.Fatalf("set stats counts: %v", err)
+	}
+}
+
+func setBestMove(t *testing.T, id uuid.UUID, variant string, score int) {
+	t.Helper()
+	tiles := `[{"letter":"c","value":3,"blank":false}]`
+	if _, err := testDB.ExecContext(context.Background(),
+		`INSERT INTO backend.account_best_move (account_id, variant, score, tiles) VALUES ($1,$2,$3,$4)
+		 ON CONFLICT (account_id, variant) DO UPDATE SET score=$3, tiles=$4`, id, variant, score, tiles); err != nil {
+		t.Fatalf("set best move: %v", err)
+	}
+}
+
+func bestMoveCount(t *testing.T, id uuid.UUID) int {
+	t.Helper()
+	var n int
+	if err := testDB.QueryRowContext(context.Background(),
+		`SELECT count(*) FROM backend.account_best_move WHERE account_id=$1`, id).Scan(&n); err != nil {
+		t.Fatalf("best move count: %v", err)
+	}
+	return n
+}
+
 func setWallet(t *testing.T, id uuid.UUID, hints int, paid bool) {
 	t.Helper()
 	if _, err := testDB.ExecContext(context.Background(),
@@ -110,8 +138,15 @@ func TestAccountMergeCore(t *testing.T) {
 
 	setStats(t, primary, 1, 0, 0, 100, 90)
 	setStats(t, secondary, 3, 1, 2, 400, 80)
+	setStatsCounts(t, primary, 100, 5)
+	setStatsCounts(t, secondary, 50, 8)
 	setWallet(t, primary, 2, false)
 	setWallet(t, secondary, 5, true)
+	// Best moves: secondary's scrabble_en (80) beats primary's (50) and is kept; secondary's
+	// scrabble_ru (30) is new to primary and carried over.
+	setBestMove(t, primary, "scrabble_en", 50)
+	setBestMove(t, secondary, "scrabble_en", 80)
+	setBestMove(t, secondary, "scrabble_ru", 30)
 
 	email := "merge-" + uuid.NewString() + "@example.com"
 	bindEmailIdentity(t, secondary, email)
@@ -153,6 +188,26 @@ func TestAccountMergeCore(t *testing.T) {
 	if mergedInto(t, secondary) != primary {
 		t.Errorf("secondary.merged_into = %s, want primary %s", mergedInto(t, secondary), primary)
 	}
+
+	// The moves/hints counters sum, and the per-variant best moves merge (the higher score
+	// per variant kept), with the secondary's best-move rows cleaned up.
+	st, err := store.GetStats(ctx, primary)
+	if err != nil {
+		t.Fatalf("get merged stats: %v", err)
+	}
+	if st.Moves != 150 || st.HintsUsed != 13 {
+		t.Errorf("primary moves/hints = %d/%d, want 150/13", st.Moves, st.HintsUsed)
+	}
+	best := map[string]int{}
+	for _, b := range st.BestMoves {
+		best[b.Variant] = b.Score
+	}
+	if len(st.BestMoves) != 2 || best["scrabble_en"] != 80 || best["scrabble_ru"] != 30 {
+		t.Errorf("primary best moves = %+v, want scrabble_en:80 + scrabble_ru:30", st.BestMoves)
+	}
+	if bestMoveCount(t, secondary) != 0 {
+		t.Error("secondary best-move rows should be deleted after merge")
+	}
 }
 
 // TestAccountMergeActiveGameConflict refuses a merge when the two share an active

docs/ARCHITECTURE.md

@@ -203,7 +203,8 @@ arrive from a platform rather than completing a mandatory registration).
   guest is promoted to durable, clearing `is_guest`).
 - **Merge** retires the account that owns the linked identity into the **current**
   account, in a single transaction (`internal/accountmerge`): statistics summed
-  (max points kept), the hint wallet summed, `paid_account` ORed, identities
+  (counters incl. moves/hints added, max points kept, and the per-variant best moves
+  merged keeping the higher-scoring play), the hint wallet summed, `paid_account` ORed, identities
   repointed, games / chat / complaints transferred, friends and blocks
   de-duplicated (friendships keep the strongest status accepted>pending>declined),
   pending invitations/codes dropped, and the secondary kept as an **audit