Add Convex backend tests for listLinkedSignalsForProject covering:
- Per-issue grouping with correct source counts
- Project membership/authz (denies non-member access)
- Cross-project signal suppression (project1 cannot see project2 signals)
- Multi-source signal source-count accuracy
- Empty project returns empty record
- Issues without linked signals are omitted from result
Make linked signals deterministically ordered newest-first within
each issue group via toSorted(createdAt descending).
