fix(markdown): resolve uploaded local image references

[wolfkill]

Summary

• resolve Markdown local image references against images already uploaded to the same knowledge base
• keep folder upload order image-first so Markdown parsing can find referenced assets
• align frontend/backend/docparser support for additional image formats including svg/webp/bmp/tiff

Fixes #952

Testing

• CGO_LDFLAGS="-lc++" go test ./internal/application/service -run "Test(IsValidFileTypeAcceptsSupportedImageFormats|ExtractMarkdownLocalImageRefs|NormalizeMarkdownLocalImagePathSkipsUnsafeRefs|MimeTypeForImageFileUsesExtension|AttachMarkdownLocalImagesUsesUploadedImageKnowledge)" -count=1
• CGO_LDFLAGS="-lc++" go test ./internal/application/repository -run TestListKnowledgeByFileNames -count=1
• CGO_LDFLAGS="-lc++" go test ./internal/infrastructure/docparser -run "TestResolveAndStore_LocalImageRefFromReadResult|TestResolveAndStore_MultipleFormats" -count=1
• npm run build-only
• Local service e2e: started the backend against local Postgres/DocReader, uploaded an image and Markdown via HTTP API, and verified the Markdown chunk rewrote images/logo.svg to a local://...svg storage URL