add proactive edit locking for issues

[MattBudz]

Summary

Adds proactive edit locking for Issues: when a user opens the edit form for an issue that another user already has open, they see a lockout screen instead of the form, with the option to go back or edit anyway. This replaces the previous approach of only warning about conflicts after save.

Testing steps

1. Log in to Dradis as a user with author access to a project, in Browser A.
2. Navigate to a project and open the Issues section.
3. Create a new issue (or use an existing one) and note its title.
4. Open a second, separate browser (or a private/incognito window) as Browser B, and log in as a different user.
5. In Browser A, click Edit on the issue.
6. Assert Browser A loads the normal "Edit issue" form.
7. In Browser B, navigate to the same issue and click Edit.
8. Assert Browser B shows a "This issue is currently being edited" lockout screen instead of the edit form.
9. Assert the lockout screen displays the name of the user editing in Browser A.
10. In Browser B, click Go back.
11. Assert Browser B returns to the issue's show page (not the edit form).
12. In Browser B, click Edit again, then click Edit anyway on the lockout screen.
13. Assert Browser B now shows the edit form for the issue.
14. In Browser A, make a change to the issue text and click Update Issue.
15. Assert Browser A shows the "Issue updated." confirmation and the change is saved.
16. In a new browser tab, log in as the Browser A user and click Edit on the same issue.
17. Assert the edit form loads normally (the lock was released after Browser A's save, so no lockout screen appears).
18. Wait for more than 24 hours since a session was acquired (or, in a test/staging environment, adjust the system clock forward), then have a new user attempt to edit a previously-locked record.
19. Assert the edit form loads normally (a stale lock older than 24 hours no longer blocks editing).

Check List

• Added a CHANGELOG entry
• Commit message has a detailed description of what changed and why.