Fix orphan pinned items after project deletion

[furtherref]

Summary

• filter and clean pinned items that point to missing issues or projects
• remove project pins transactionally when deleting projects
• publish pin deletion events so sidebar pin caches refresh
• add handler regression tests for project pin cleanup and historical orphan pins

Test plan

• git diff --check
• cd server && go test ./internal/handler -run 'Test(DeleteProjectRemovesPins|ListPinsOmitsMissingProjectPins)' -count=1 -v (local DB unavailable, handler suite skipped with database not reachable)\n- git push pre-push hook: lint, typecheck, test, build passed