feat: add DELETE endpoint to notifications API (closes #2411)

[Rudrasamadhiya]

Summary

The notifications API had GET (fetch) and PATCH (mark-as-read) handlers but no DELETE endpoint, making it impossible for clients to dismiss or clear notifications. This PR adds a DELETE handler that deletes all notifications for the authenticated user.

Closes #2411

Type of Change

• New feature (adds missing REST endpoint)

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus