feat: add tenant audit provenance events

[seonghobae]

Summary

• add a tenant-scoped audit_event_record with opaque service actor defaults, correlation UUID, source authority/version, event identity, and SHA-256 event digest
• enforce append-only audit rows with PostgreSQL RLS plus a mutation-rejecting trigger
• record successful assessment import, completion publication, credential issue, and credential revocation transitions in the same transaction
• do not duplicate assessment, learner, credential, or provider payloads; idempotent assessment retries and repeated revocation do not create duplicate audit events
• extend the exact-head API smoke gate to assert 16 tenant policies, four audit events, and mutation rejection
• document the current provenance evidence and remaining correlation propagation/export gap

Evidence

• real PostgreSQL 18.4 API E2E on a NOSUPERUSER application role: assessment retry returned the same evidence ID; audit count was 4 with four distinct correlation IDs; cross-tenant audit visibility was 0; update mutation was rejected by the append-only trigger
• cargo test --workspace --all-targets --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• RUSTDOCFLAGS=-Dwarnings cargo doc --workspace --no-deps --locked
• cargo build --bin lms_api --locked
• cargo fmt --all -- --check
• actionlint
• git diff --check

Gate

This is stacked on PR #12 and remains unmerged product evidence until an independent current-head review and required Checks pass. Merge only after re-fetching exact head 4fafb5d and confirming protected rules.

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[seonghobae]

Current-head review request for 4fafb5d. Please independently verify the append-only audit trigger, tenant RLS behavior under the application role, transaction atomicity for assessment/completion/credential lifecycle events, and the exact-head quality assertions. This is a COMMENT only; no approval or protected-merge bypass is requested.

[seonghobae]

Current-head review request for e31ed03. Re-verify the exact-head diff, required Checks, tenant/security boundaries, and any valid review findings after the shared Rust component-install fix. COMMENT only; no approval or protected-merge bypass is requested.