feat(oo): native document conversion (docs pdf/presigned)

- lib: PresignedURI, SignJWT (HS256, stdlib), ConvertDocument (DocumentServer
  /converter; legacy /ConvertService.ashx), DownloadURLTo
- oo docs pdf FILE_ID|PATH...: OO file ids or local files (temp upload to
  --folder, convert, download, cleanup) -> PDF/other
- oo docs presigned FILE_ID
- docs base from $ONLYOFFICE_DOCS_URL else $ONLYOFFICE_URL/ds-vpath; JWT secret
  from $ONLYOFFICE_DS_SECRET (DocumentServer CoAuthoring secret)
- test: SignJWT

convert_test.go

@@ -0,0 +1,24 @@
+package onlyoffice
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSignJWT(t *testing.T) {
+	payload := map[string]any{"url": "u", "outputtype": "pdf"}
+	tok, err := SignJWT("secret", payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n := len(strings.Split(tok, ".")); n != 3 {
+		t.Fatalf("JWT must have 3 parts, got %d", n)
+	}
+	tok2, _ := SignJWT("secret", payload)
+	if tok != tok2 {
+		t.Fatal("SignJWT must be deterministic for identical input")
+	}
+	if _, err := SignJWT("", payload); err == nil {
+		t.Fatal("expected error for empty secret")
+	}
+}