Support re-import for recipes without a source URL

Recipes imported by drag & drop / file-picker record no source_path and
were rejected by re-import. Fall back to the recipe's own saved image,
which still carries the original embedded generation metadata.

Re-import now re-parses that original metadata instead of the appended
recipe JSON block, so parser upgrades produce fresh results. The
already-optimized preview image is kept verbatim: only its WebP EXIF
chunk is rewritten in place to replace the recipe metadata block, and
the recipe JSON is rewritten with the new analysis plus carried-over
user edits.

py/utils/exif_utils.py

@@ -348,8 +348,14 @@ class ExifUtils:
             return image_path
             
     @staticmethod
-    def append_recipe_metadata(image_path, recipe_data) -> str:
-        """Append recipe metadata to an image's EXIF data"""
+    def append_recipe_metadata(image_path, recipe_data, pixel_preserving=False) -> str:
+        """Append recipe metadata to an image's EXIF data
+
+        When ``pixel_preserving`` is True (and the image is a WebP) only the
+        EXIF container is rewritten at the byte level, so the preview pixels
+        are never re-encoded. Local re-import uses this because its source is
+        the recipe's own already-optimized preview image.
+        """
         try:
             if image_path:
                 ext = os.path.splitext(image_path)[1].lower()
@@ -417,13 +423,71 @@ class ExifUtils:
             
             # Append to existing metadata or create new one
             new_metadata = f"{metadata} \n {recipe_metadata_marker}" if metadata else recipe_metadata_marker
-            
+
+            # Write back to the image. Re-import keeps the already-optimized
+            # preview pixels untouched and updates only the WebP EXIF chunk
+            # instead of re-encoding the whole image.
+            if pixel_preserving and image_path.lower().endswith(".webp"):
+                metadata_fields = ExifUtils._load_structured_metadata(image_path)
+                metadata_fields["parameters"] = new_metadata
+                exif_bytes = ExifUtils._build_exif_bytes(metadata_fields)
+                with open(image_path, "rb") as file_obj:
+                    image_bytes = file_obj.read()
+                try:
+                    updated = ExifUtils._replace_webp_exif(image_bytes, exif_bytes)
+                except ValueError:
+                    # Container without an EXIF chunk; fall back to re-encoding.
+                    return ExifUtils.update_image_metadata(image_path, new_metadata)
+                with open(image_path, "wb") as file_obj:
+                    file_obj.write(updated)
+                return image_path
+
             # Write back to the image
             return ExifUtils.update_image_metadata(image_path, new_metadata)
         except Exception as e:
             logger.error(f"Error appending recipe metadata: {e}", exc_info=True)
             return image_path
 
+    @staticmethod
+    def _replace_webp_exif(image_bytes: bytes, exif_bytes: bytes) -> bytes:
+        """Replace the EXIF chunk of a WebP file without re-encoding pixels."""
+        if image_bytes[:4] != b"RIFF" or image_bytes[8:12] != b"WEBP":
+            raise ValueError("Not a WebP file")
+        # The WebP EXIF chunk stores raw TIFF data; strip the JPEG-style
+        # "Exif\\0\\0" prefix that piexif.dump may prepend.
+        tiff = exif_bytes[6:] if exif_bytes[:6] == b"Exif\x00\x00" else exif_bytes
+
+        out = bytearray(image_bytes[:12])
+        pos = 12
+        exif_payload = None
+        while pos + 8 <= len(image_bytes):
+            fourcc = image_bytes[pos : pos + 4]
+            size = struct.unpack("<I", image_bytes[pos + 4 : pos + 8])[0]
+            chunk_data = image_bytes[pos + 8 : pos + 8 + size]
+            pad = size % 2
+            if fourcc == b"EXIF":
+                exif_payload = tiff
+            else:
+                out += (
+                    fourcc
+                    + struct.pack("<I", size)
+                    + chunk_data
+                    + (b"\x00" * pad)
+                )
+            pos += 8 + size + pad
+
+        if exif_payload is None:
+            raise ValueError("WebP has no EXIF chunk")
+
+        out += (
+            b"EXIF"
+            + struct.pack("<I", len(exif_payload))
+            + exif_payload
+            + (b"\x00" * (len(exif_payload) % 2))
+        )
+        out[4:8] = struct.pack("<I", len(out) - 8)
+        return bytes(out)
+
     @staticmethod
     def remove_recipe_metadata(user_comment):
         """Remove recipe metadata from user comment"""