feat: add local image enhancement and lossless mod assets

backend/app/converter/assets.py

@@ -75,6 +75,22 @@ def _image(src: Path, tmp: Path) -> tuple[Path, str]:
         return out, ".webp"
 
 
+def lossless_image(src: Path, tmp: Path) -> tuple[Path, str]:
+    """Validate a static PNG/WebP and preserve its pixels (and original bytes when possible)."""
+    with Image.open(src) as img:
+        if img.format not in ("PNG", "WEBP") or getattr(img, "is_animated", False):
+            raise ValueError("Нужен статичный PNG или WebP")
+        img.load()  # Decode fully: a file header alone is not validation.
+        if max(img.size) > WEBP_MAX:
+            out = tmp.with_suffix(".png")
+            img.save(out, "PNG")
+            return out, ".png"
+        ext = ".png" if img.format == "PNG" else ".webp"
+        out = tmp.with_suffix(ext)
+        shutil.copyfile(src, out)
+        return out, ext
+
+
 def _ffmpeg(src: Path, out: Path, args: list[str]) -> Path:
     proc = subprocess.run(
         ["ffmpeg", "-nostdin", "-hide_banner", "-loglevel", "error", "-y", "-i", str(src),