[CT-39] Wire event.rs into the crate as a real audit module

[mftee]

Overview

event.rs defines a fully-implemented Event struct (with new(), with_metadata(), to_json(), from_json()) and has its own passing unit tests — but lib.rs's module declarations (pub mod cache; pub mod config; pub mod hash_validator; pub mod metrics; pub mod rate_limit; pub mod stellar;) never include pub mod event;, so this file is not part of the compiled crate at all. It also references crate::error::Result and crate::error::AuditError, but no error.rs file exists anywhere in contract/src — so even if declared, it would not compile as-is.

Files

• contract/src/lib.rs
• contract/src/event.rs
• contract/src/error.rs (new)

Acceptance Criteria

• Create contract/src/error.rs defining AuditError (at minimum a SerializationError variant, matching event.rs's usage) and a crate-level Result<T> alias
• Add pub mod error; and pub mod event; to lib.rs
• Wire Event::new() into at least the verify/submit/revoke/transfer handlers to emit an audit event per request, persisted via the existing CacheBackend (or a dedicated append-only store)
• cargo build and cargo test pass with event.rs included in the compiled crate
• Add an admin-facing GET /audit endpoint to read back emitted events, or explicitly note it as a follow-up if out of scope for this issue