Nityam/fix private assets authorization

[Nixxx19]

Issue:

Fixes #3904

The getProjectAsset() endpoint was serving project assets (files, images) for any project ID without checking the project's visibility setting. This security vulnerability allowed anyone who knew or guessed a project ID to access Private project assets, even when logged out or as a different user. While project listings correctly filter by visibility, this endpoint did not enforce the same authorization checks.

Changes:

Added authorization check to getProjectAsset():

• Added visibility and ownership validation before serving assets
• Private projects are now only accessible to their owners
• Unauthenticated users are blocked from accessing private project assets (returns 403)
• Non-owners are blocked from accessing private project assets (returns 403)
• Public projects remain accessible to everyone (no breaking changes)

Security improvements:

• Prevents unauthorized access to private project assets
• Protects all vulnerable routes: /full/:project_id/*, /embed/:project_id/*, /:username/sketches/:project_id/*, /present/:project_id/*
• Uses proper ObjectId comparison for ownership verification
• Follows the same authorization pattern as other protected endpoints in the codebase

Added comprehensive test coverage:

• Created test suite with 8 test cases covering all scenarios
• Tests verify: unauthenticated access blocking, non-owner blocking, owner access, public project access, and error handling
• All tests pass successfully

Files changed:

• server/controllers/project.controller.js: Added visibility and ownership check to getProjectAsset function
• server/controllers/project.controller/__test__/getProjectAsset.test.js: Added comprehensive test suite

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #3904
• meets the standards outlined in the accessibility guidelines