ARCH-15 Batch 3: Extract command idempotency types and constants

[chuanman2707]

Parent roadmap: #133
Depends on: ARCH-14

Goal

Start splitting command_idempotency.rs with a low-risk types/constants extraction.

Scope

• mhm/src-tauri/src/command_idempotency.rs
• New focused module for command safety types/constants

Non-goals

• Do not change transaction logic.
• Do not change SQL.
• Do not change replay semantics.
• Do not change outbox behavior.

Acceptance Criteria

• Types/constants move to a focused module.
• Public API remains stable for callers.
• Tests pass with no behavior change.

Validation

cd mhm/src-tauri && cargo test command_idempotency
cd mhm/src-tauri && cargo clippy --all-targets -- -D warnings

Evidence

• command_idempotency.rs defines request/idempotency fields and lock-key related state in the same large file.

[chuanman2707]

Completed by PR #168. Evidence: command idempotency contract/type surfaces extracted into command_idempotency/types.rs without behavior change.