fix: add security headers to file serving endpoints to prevent stored XSS (#18857)

## Summary

- File serving endpoints (`GET file/:fileFolder/:id` and `GET
public-assets/...`) were piping S3/local file streams directly to the
response without any HTTP headers, allowing a stored XSS attack via
uploaded HTML files rendered inline on the CRM origin.
- Adds `Content-Type`, `Content-Disposition`, and
`X-Content-Type-Options: nosniff` headers to all file serving responses.
Only known-safe MIME types (images, PDF, plain text, audio, video) are
served inline; everything else (HTML, SVG, XML, etc.) forces
`Content-Disposition: attachment` to trigger download instead of
rendering.
- New `setFileResponseHeaders` utility with an explicit allowlist of
inline-safe MIME types.

## Test plan

- [x] Unit tests pass (9 tests including 2 new ones: header assertions
and attachment-disposition for HTML)
- [x] Lint clean (`lint:diff-with-main`)
- [x] Typecheck clean (`nx typecheck twenty-server`)
- [ ] Manual: upload an HTML file via `uploadWorkflowFile`, access the
returned URL — should download instead of rendering
- [ ] Manual: upload a PNG image, access the URL — should render inline
with correct `Content-Type: image/png`
- [ ] Manual: verify `X-Content-Type-Options: nosniff` header is present
on all file responses


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---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Etienne <etiennejouan@users.noreply.github.com>
Co-authored-by: Etienne <45695613+etiennejouan@users.noreply.github.com>

packages/twenty-server/src/engine/core-modules/application/application-install/application-install.service.ts

@@ -178,6 +178,8 @@ export class ApplicationInstallService {
         );
       }
 
+      // TODO: mimeType should be defined, default to application/octet-stream, which won't be displayed
+      // inline by the browser (forced download) due to Content-Disposition security headers.
       await this.fileStorageService.writeFile({
         sourceFile: content,
         mimeType: undefined,