[bug] Deleted GitHub issues remain in mirror after issues.deleted webhook

[pekar9781]

Description

IssueHandler upserts issue rows for opened, edited, closed, and labeled events but does not handle issues.deleted. When a maintainer deletes a GitHub issue, the mirror keeps the last-known row and miners API issue queries can still return it.

CommentHandler and ReviewCommentHandler already delete rows on deleted; issues are missing the same behavior.

Steps to Reproduce

1. Register a repo in the mirror and receive an issues.opened webhook for issue fix: backfill nested pagination #10.
2. Confirm the issue row exists in issues and appears in miners issue endpoints.
3. Delete issue fix: backfill nested pagination #10 on GitHub (issues.deleted webhook).
4. IssueHandler upsert path never runs a delete; the stale row remains queryable.

Expected Behavior

Deleted issues are removed from the mirror (or otherwise excluded from miners responses).

Actual Behavior

Deleted issues remain stored with their last-known state.

Environment

• OS: Linux
• Runtime/Node version: Node 20