fix: qualify PM Inbox timestamps in PostgreSQL upsert

[CSJ-PJT]

Scope

• Qualify PM Inbox timestamp references in the PostgreSQL upsert.
• Add a PostgreSQL 16 integration test task for PM Inbox transitions.

Validation passed

• archiveos-ai: clean test bootJar
• archiveos-ai: postgresIntegrationTest against an isolated pgvector/pgvector:pg16 instance
• Backend test, typecheck, build
• Frontend test, smoke, production build
• RC Compose config and image build
• Isolated container health
• PM Inbox boundary smoke: Public POST 401, PM POST 403, Admin session 200
• Secret scan and git diff --check

Isolated fresh-DB limitation

A full Admin HTTP mutation against an empty disposable Compose database still returns 500 before reaching the upsert because PM Inbox aggregation reads legacy tables such as daily_reports that are absent from the current Flyway-only schema. No migration, data correction, or production database change is included in this PR. The timestamp upsert itself passes actual PostgreSQL 16 transition, idempotency, metadata-merge, and rollback tests.

No environment changes, main merge, OCI branch change, or operational data mutation are included.