Fix: Skip non-Image items when saving character sheet stages

The stages dictionary contains both images and metadata (prompts, etc).
When saving stages to disk, the code was trying to save prompt strings
as images, which caused ensure_pil_image() to treat them as file paths
and throw "Image path not found: {prompt text}" exceptions.

This fix adds isinstance(item, Image.Image) check to only save actual
PIL Images, skipping prompt strings and other metadata.

Now character sheets display successfully even when safety filters
trigger retries or individual stages fail with placeholders.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

character_forge_image/services/character_forge_service.py

@@ -1132,11 +1132,11 @@ class CharacterForgeService:
             metadata=metadata
         )
 
-        # Save individual stages
-        for stage_name, image in stages.items():
-            if stage_name != 'character_sheet':
+        # Save individual stages (skip prompts and other metadata)
+        for stage_name, item in stages.items():
+            if stage_name != 'character_sheet' and isinstance(item, Image.Image):
                 save_image(
-                    image=image,
+                    image=item,
                     directory=char_dir / "stages",
                     base_name=f"{safe_name}_{stage_name}",
                     metadata=None