feat(migrations): partition audit_logs and pii_access_audit_logs by month

[Gabbydunkk]

Summary

Partitions the two audit tables so they scale with the transaction_logs approach already in the codebase:

• audit_logs — RANGE-partitioned on created_at
• pii_access_audit_logs — RANGE-partitioned on accessed_at

Monthly partitions follow the same zero-downtime pattern established by 20260723_partition_logs: the existing table is renamed to *_legacy and re-attached as the DEFAULT partition (no data copy), monthly partitions are pre-created (past, current, next 3 months), and a BEFORE INSERT route trigger creates the partition for a row's timestamp on demand so an audit write can never fail for want of a partition. The append-only immutability triggers are recreated on the partitioned parents, where PostgreSQL ≥ 14 fires them for every current and future partition.

Verification (Postgres 16)

Run against a real Postgres 16 container:

• Up: all 85 migrations apply cleanly on a fresh DB; both tables are partitioned with *_legacy + monthly partitions; inserts route into the correct monthly partition; UPDATE/DELETE are blocked (Audit logs are immutable and cannot be modified or deleted).
• Down: monthly partitions are drained into the legacy table before the parent is dropped — rows written after the up migration survive the rollback — and the original primary keys, indexes, and immutability triggers are restored exactly.
• Re-up: idempotent; existing data is preserved through the cycle.

Changes

• migrations/20260826_partition_audit_logs.sql — up migration (both tables, helpers, route + immutability triggers, indexes)
• migrations/20260826_partition_audit_logs.down.sql — rollback (drain partitions, restore original schema, data-safe)

Closes #1858

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