feat(backend): slice 8 — export scene to mxdrw

- internal/exporter: pure BuildScene + 21-char base62 IDs, port ellipses,
  device rect+text pairs, IO diamonds, arrow bindings, legend texts.
  Bundles intentionally omitted per design §4.1.
- internal/db: PersistExcalidrawIDs idempotent updater per project.
- internal/server: POST /api/projects/:pid/sync/export — loads snapshot,
  mints/reuses excalidraw_ids, PUTs scene to mxdrw with bearer auth.
  Returns viewer URL + element_count + mxdrw response.

Roundtrip hand-tested against mxdrw.msbls.de: scene saved, IDs stable
across re-exports.

internal/db/excalidraw_ids.go

@@ -0,0 +1,60 @@
+package db
+
+import (
+	"database/sql"
+)
+
+// PersistExcalidrawIDs writes the assignments returned by the exporter
+// back onto the corresponding rows. Idempotent: only updates rows whose
+// excalidraw_id is currently NULL (the first export "owns" the id; later
+// exports reuse it so mxdrw's collab cursors / undo history survive).
+//
+// Caller passes one map per kind; keys are the in-project row ids,
+// values are the 21-char Excalidraw element ids the exporter minted.
+func (s *Store) PersistExcalidrawIDs(projectID int64,
+	frames, devices, ports, ios, cables map[int64]string,
+) error {
+	tx, err := s.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	if err := updateExIDs(tx, "frames", projectID, frames); err != nil {
+		return err
+	}
+	if err := updateExIDs(tx, "devices", projectID, devices); err != nil {
+		return err
+	}
+	if err := updateExIDs(tx, "ports", projectID, ports); err != nil {
+		return err
+	}
+	if err := updateExIDs(tx, "io_markers", projectID, ios); err != nil {
+		return err
+	}
+	if err := updateExIDs(tx, "cables", projectID, cables); err != nil {
+		return err
+	}
+	return tx.Commit()
+}
+
+func updateExIDs(tx *sql.Tx, table string, projectID int64, m map[int64]string) error {
+	if len(m) == 0 {
+		return nil
+	}
+	stmt, err := tx.Prepare(
+		`UPDATE ` + table + `
+		 SET excalidraw_id = ?
+		 WHERE id = ? AND project_id = ? AND excalidraw_id IS NULL`,
+	)
+	if err != nil {
+		return err
+	}
+	defer stmt.Close()
+	for id, exID := range m {
+		if _, err := stmt.Exec(exID, id, projectID); err != nil {
+			return err
+		}
+	}
+	return nil
+}