feat: split audit service and tighten contract safeguards

[meetdarc-tech]

Summary

This PR addresses the backend and smart-contract issues identified in the audit service split, admin access-control review, type utility cleanup, and contract upgrade/versioning documentation work.

What changed

Backend audit separation

• Split the audit service into read-oriented and write-oriented responsibilities to keep query/export/report flows separate from log mutation logic.
• Kept the shared in-memory store behind a single backing state so read-side and write-side access remain consistent without mixing responsibilities.
• Clarified the append-only behavior and retention/alert bookkeeping in the write-only layer.

Smart-contract admin safeguards

• Reviewed privileged function access control and documented the invariant that all admin-only mutations must pass a pre-write admin check.
• Added explicit non-admin rejection coverage for privileged operations so the contract enforces role boundaries before mutation.

Type utility cleanup

• Centralized enum conversion helper logic in the utility module and kept the type definitions focused on the actual type declarations.
• Reduced overlap between conversion helpers and enum definitions while preserving the intended API surface.

Upgrade/versioning clarity

• Documented the supported upgrade path and state-preserving migration model.
• Clarified that contract version transitions should preserve existing stored state and reject unsupported or unsafe version changes before applying new implementation metadata.

Why

These changes reduce accidental coupling between read and write concerns, make the admin control model easier to reason about, eliminate duplicated conversion logic, and document the safe upgrade path so future contract changes stay compatible with persisted state.

Scope

This PR is intentionally scoped to the issue areas above and does not widen the project surface beyond the relevant backend and contract modules.

closes #1084
closes #1109
closes #1110
closes #1111

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