docs: consolidate documentation into organized structure

- Move 12 markdown files from root to docs/ subdirectories
- Organize documentation by category:
  • docs/troubleshooting/ (1 file)  - Technical troubleshooting guides
  • docs/deployment/      (4 files) - Deployment and security documentation
  • docs/guides/          (3 files) - Feature-specific guides
  • docs/planning/        (4 files) - Planning and improvement proposals

Root directory cleanup:
- Reduced from 16 to 4 markdown files in root
- Only essential project files remain:
  • CLAUDE.md (AI instructions)
  • README.md (Main project readme)
  • CLEANUP_PLAN.md (Current cleanup plan)
  • SRC_STRUCTURE_IMPROVEMENTS.md (Structure improvements)

This improves:
✅ Documentation discoverability
✅ Logical organization by purpose
✅ Clean root directory
✅ Better maintainability

src/Domain/Media/ImageSourceSetGenerator.php

@@ -155,7 +155,7 @@ final readonly class ImageSourceSetGenerator
 
         // Wenn keine Varianten gefunden wurden, das Originalbild verwenden
         return new ImageVariant(
-            imageId: $image->ulid,
+            imageId: (string) $image->ulid,
             variantType: 'original',
             size: 'original',
             format: pathinfo($image->filename, PATHINFO_EXTENSION),
@@ -164,7 +164,7 @@ final readonly class ImageSourceSetGenerator
             width: $image->width,
             height: $image->height,
             filename: $image->filename,
-            path: $image->path,
+            path: $image->path->toString(),
         );
     }
 }