fix: harden Kanban board reads

[AIEngineerX]

Summary

• verify Kanban SQLite read connections before returning them
• fall back to immutable read-only mode when normal read-only opens but first query fails
• add a regression test for existing board DB reads

Verification

• python3 -m unittest tests.test_privacy_contract -v
• npm run verify
• npm run test:security
• npm run test:performance
• npm run test:compat
• npm audit --audit-level=moderate
• live collect_kanban check: stderr empty; default board read with no error