Fix team-owned invoice edit permissions

[Fanwize]

Description

Fixes a permission bug in the invoice edit endpoint for team-owned invoices.

The endpoint previously performed manual permission checks against both the active team and the invoice user. For team-owned invoices, invoice.organization is set while invoice.user may be empty. This meant that a user logged in as the owning team could still be incorrectly rejected with a 403 response.

This PR replaces the duplicated manual permission check with the existing invoice.has_access(request.user) helper, which already handles both personal and team-owned invoices.

Regression tests were added for:

• allowing a team member to edit an invoice owned by their active team
• rejecting a user who is not currently acting in the matching team context

No new dependencies are required.

Checklist

• Ran the Black Formatter and
  djLint-er on any new code
• Made any changes or additions to the documentation where required
• Changes generate no new warnings/errors
• New and existing unit tests pass locally with my
  changes

What type of PR is this?

• 🐛 Bug Fix

Added/updated tests?

• 👍 yes

Related PRs, Issues etc

• Related Issue: N/A