Add admin audit query endpoint with retention pruning

[pavsoss]

Summary

Part 2 of 2 for #1023. Builds on the tamper-evident audit store by making the
trail usable operationally.

Depends on Part 1 (feat/1023-audit-store).

• GET /audit (admin only) returns stored records newest-first, with
  exact-match filters (actor, action, resource), an inclusive ISO-8601 time
  window, and limit/offset pagination. The response also reports whether the
  hash chain still verifies, so integrity status travels with the data.
• Access reuses the existing privileged-route pattern: the service-to-service
  internal secret plus the forwarded caller identity and admin role — no new
  auth scheme.
• A configurable retention window (AUDIT_RETENTION_DAYS) prunes records past
  the cutoff and re-seals the surviving chain so verification continues to pass.
  Pruning is the only sanctioned mutation of the otherwise append-only trail.
• The new endpoint is documented in the OpenAPI spec.

Parts

• Part 1 of 2 ([Feature]: Tamper-evident, queryable audit trail for the Flask ML API #1023) — tamper-evident audit store (dependency).
• Part 2 of 2 (this PR) — query endpoint, retention, and docs.

Test plan

Coverage for filter composition, the inclusive time window, pagination and
limit clamping, and for the retention cutoff — including that pruning removes
only expired records, returns the deleted count, leaves the retained trail
verifying, and is a no-op for a non-positive window.

Closes #1023

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