# Secure File Downloads

## Objective

Serve private files without turning storage URLs into permanent public links.

Authorization checked at download time, followed by a short-lived signed URL that the storage layer serves directly.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Route every download through an app endpoint that checks the current user's authorization for that specific file, then redirects to a signed URL with a short expiry.
2. Keep the expiry measured in minutes. A signed link is a bearer credential and will end up in browser history, chat messages, and referrer headers.
3. Set Content-Disposition to attachment and a conservative Content-Type for user-uploaded files so the browser downloads rather than renders them.
4. Let the storage layer handle range requests and large transfers; the app issues the URL and gets out of the way.
5. Do NOT rely on an unguessable storage path as the access control. Unguessable is not private — it is a permanent public link nobody has found yet.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Authorization must be re-checked at every download, not cached from when the page listing the file was rendered.
- Removing a user from a workspace must break their outstanding signed URLs faster than those URLs expire, or the expiry is too long.
- Filenames from users can contain newlines, quotes, and non-ASCII characters — sanitise and encode them or the header is an injection point.
- SVG and HTML uploads execute script when served inline. Force attachment disposition, or serve user content from a separate origin.
- A resumed or partially failed download must not require a fresh authorization round-trip mid-transfer.
- Log who downloaded what and when; for private files that record is often the point.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Every download re-checks authorization for the specific file at request time.
- [ ] Signed URLs are short-lived and never stored or shared as permanent links.
- [ ] User-uploaded files are served with attachment disposition and a safe content type.
- [ ] Filenames are sanitised and encoded in headers.
- [ ] Large files and range requests are served by storage, not proxied through the app.
- [ ] Access changes invalidate outstanding links promptly.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.