feat: add audit export operational receipts

[seonghobae]

What changed

• preserve the existing tenant audit-export JSON array response
• add page count, ordered receipt digest, and last-event keyset cursor headers
• define the receipt encoding in ADR-0005 without copying provider payloads
• verify both first and subsequent pages in the exact-head PostgreSQL/API smoke path

Why

Operators and incident/reconciliation consumers need a compact, independently recomputable record of which audit page they received and where to resume. The receipt remains caller-retained response evidence; it is not a second audit authority or a persisted source-payload copy.

Validation

• real PostgreSQL 18.4 disposable database and API
• first page 2, remaining page 3, receipt digest recomputation, next-cursor header reuse
• partial cursor HTTP 400
• cross-tenant export returns empty
• actionlint
• pinned Rust fmt, workspace tests (6), Clippy, rustdoc, docs contract, and diff checks

Stacked on PR #19 exact head a718bfe. Independent review, exact-head protected Checks, and protected merge remain required; no self-approval or protection bypass was used.

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[seonghobae]

@opencode-agent Review only exact current head 19c16d2 against base a718bfe; do not update, approve, merge, or bypass protections. Focus on backward-compatible audit-export receipt headers, canonical UUID digest encoding, cursor header reuse, tenant isolation, and payload exclusion.

[coderabbitai]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.