#769 MEDIUM: claim_task() should filter by tenant_id for multi-tenant isolation

Issue #3 from audit: claim_task() SQL function doesn't filter by tenant_id. Tasks have tenant_id column but workers can claim from any tenant in shared queue. Requires: 1) Add p_tenant_id parameter to claim_task(), 2) Add WHERE tenant_id filter, 3) Update all Python callers. Deferred to Phase 2 due to migration complexity.