feat(db): add AppendSteps, GetStepsSince, GetDocVersion for step log

internal/db/db.go

···
 	_, err := db.Exec(`DELETE FROM invites WHERE token = ?`, token)
 	return err
 }
+
+// --- Document Steps (prosemirror-collab) ---
+
+type StepRow struct {
+	Version int
+	JSON    string
+}
+
+func (db *DB) GetDocVersion(docRKey string) (int, error) {
+	var v int
+	err := db.QueryRow(
+		`SELECT COALESCE(MAX(version), 0) FROM doc_steps WHERE doc_rkey = ?`, docRKey,
+	).Scan(&v)
+	if err != nil {
+		return 0, fmt.Errorf("GetDocVersion: %w", err)
+	}
+	return v, nil
+}
+
+func (db *DB) AppendSteps(docRKey string, clientVersion int, stepsJSON []string, clientID string) (int, error) {
+	tx, err := db.Begin()
+	if err != nil {
+		return 0, fmt.Errorf("AppendSteps begin: %w", err)
+	}
+	defer tx.Rollback()
+
+	var current int
+	tx.QueryRow(`SELECT COALESCE(MAX(version), 0) FROM doc_steps WHERE doc_rkey = ?`, docRKey).Scan(&current)
+	if current != clientVersion {
+		return 0, fmt.Errorf("version conflict: server=%d client=%d", current, clientVersion)
+	}
+
+	for i, stepJSON := range stepsJSON {
+		version := clientVersion + i + 1
+		_, err := tx.Exec(
+			`INSERT INTO doc_steps (doc_rkey, version, step_json, client_id) VALUES (?, ?, ?, ?)`,
+			docRKey, version, stepJSON, clientID,
+		)
+		if err != nil {
+			return 0, fmt.Errorf("AppendSteps insert v%d: %w", version, err)
+		}
+	}
+
+	if err := tx.Commit(); err != nil {
+		return 0, fmt.Errorf("AppendSteps commit: %w", err)
+	}
+	return clientVersion + len(stepsJSON), nil
+}
+
+func (db *DB) GetStepsSince(docRKey string, sinceVersion int) ([]StepRow, error) {
+	rows, err := db.Query(
+		`SELECT version, step_json FROM doc_steps WHERE doc_rkey = ? AND version > ? ORDER BY version ASC`,
+		docRKey, sinceVersion,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("GetStepsSince: %w", err)
+	}
+	defer rows.Close()
+	var result []StepRow
+	for rows.Next() {
+		var r StepRow
+		if err := rows.Scan(&r.Version, &r.JSON); err != nil {
+			return nil, err
+		}
+		result = append(result, r)
+	}
+	return result, rows.Err()
+}

internal/db/db_steps_test.go

···
+package db_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/limeleaf/diffdown/internal/db"
+)
+
+func openTestDB(t *testing.T) *db.DB {
+	t.Helper()
+	f, err := os.CreateTemp("", "diffdown-test-*.db")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.Remove(f.Name()) })
+	f.Close()
+
+	database, err := db.Open(f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	db.SetMigrationsDir("../../migrations")
+	if err := database.Migrate(); err != nil {
+		t.Fatal(err)
+	}
+	return database
+}
+
+func TestGetDocVersion_Empty(t *testing.T) {
+	d := openTestDB(t)
+	v, err := d.GetDocVersion("rkey1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if v != 0 {
+		t.Errorf("expected version 0 for new doc, got %d", v)
+	}
+}
+
+func TestAppendSteps_IncreasesVersion(t *testing.T) {
+	d := openTestDB(t)
+	steps := []string{`{"stepType":"replace"}`, `{"stepType":"replace"}`}
+	newVersion, err := d.AppendSteps("rkey1", 0, steps, "client-a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if newVersion != 2 {
+		t.Errorf("expected version 2, got %d", newVersion)
+	}
+}
+
+func TestAppendSteps_VersionConflict(t *testing.T) {
+	d := openTestDB(t)
+	_, err := d.AppendSteps("rkey1", 0, []string{`{}`}, "client-a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Submit again from clientVersion=0 (stale) — must fail.
+	_, err = d.AppendSteps("rkey1", 0, []string{`{}`}, "client-b")
+	if err == nil {
+		t.Fatal("expected conflict error, got nil")
+	}
+}
+
+func TestGetStepsSince(t *testing.T) {
+	d := openTestDB(t)
+	steps := []string{`{"a":1}`, `{"a":2}`, `{"a":3}`}
+	d.AppendSteps("rkey1", 0, steps, "client-a")
+
+	rows, err := d.GetStepsSince("rkey1", 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(rows) != 2 {
+		t.Errorf("expected 2 steps since v1, got %d", len(rows))
+	}
+	if rows[0].JSON != `{"a":2}` {
+		t.Errorf("unexpected step: %s", rows[0].JSON)
+	}
+}