P2: make ASTER readiness validate the complete migrated schema

[tuzuminami]

Problem

ASTER readiness currently checks only personas and persona_versions; a partial database can return /ready 200 while compiled bundles, idempotency, audit, or plugin operations fail.

Acceptance Criteria

• Validate the packaged migration ledger/checksums and all required tables and columns.
• Cover partial schema states including missing compiled bundle, idempotency, and plugin migrations.
• Make /ready return 503 until the full durable schema is usable.
• Add an integration test that executes a minimal mutation after readiness succeeds.