Add Flyway V68 migration that provisions a read-only PostgreSQL role
scoped to audit_log, documents, and transcription_blocks. The role's
password is injected via the new ${grafanaDbPassword} Flyway placeholder,
which FlywayConfig reads from the GRAFANA_DB_PASSWORD env var. The
migration is idempotent: CREATE on first run, ALTER on re-run.
Adds a Testcontainers integration test asserting positive grants on the
three intended tables and a negative grant on app_users (NFR-SEC-01).
Refs #651.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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