Add Idempotency-Key support on /api/health mutations

[arisu6804]

Closes #665

Summary

POST /api/health/mutations writes an audit-log row on every call, recording a before/after state transition (mode/maintenance). A client retry after a network blip (e.g. a response that timed out client-side after the write actually succeeded server-side) silently creates a duplicate mutation record, with no way to detect or dedupe it.

This repo already has a general-purpose Idempotency-Key middleware (src/middleware/idempotency.ts), applied globally to /api POST/PATCH routes in src/index.ts — but that global registration happens after app.use("/api/health", healthRouter), so it never actually runs for this route (Express matches middleware in registration order; the request is fully handled by healthRouter before ever reaching the later idempotency mount).

Fix: apply idempotency directly as route-level middleware on POST /mutations in src/routes/health.ts, so retried requests with the same Idempotency-Key + body replay the original response instead of re-executing the mutation (and re-writing the audit log). Requests without the header are unaffected — the middleware is a no-op when no key is present.

Tests

Added tests/healthIdempotency.test.ts (5 cases): replay on a repeated key+body (audit log written exactly once across two identical requests), 409 conflict when the same key is reused with a different body, 400 for a malformed key, independent processing when no key header is sent, and a schema sanity check. Uses a small stateful in-memory map for the db mock (mirroring the existing tests/authIdempotency.test.ts pattern) so persist-then-replay round trips can be exercised without a real database.

Note on src/middleware/timeout.ts: while writing these tests I found that requestTimeout's req.on("close", () => abort()) handler fires under supertest before the response finishes, causing abortableRace to reject in-flight promises — this reproduces in total isolation (a two-line Express app with just requestTimeout + abortableRace, no idempotency or health-route code involved) and is unrelated to this change (confirmed git diff main -- src/middleware/timeout.ts is empty). It's out of scope for this issue since requestTimeout is shared by most routes in this codebase and warrants its own investigation; I stubbed both requestTimeout and abortableRace as pass-throughs in this test file so the idempotency suite isn't flaky because of it.

Verification

• npx jest tests/healthIdempotency.test.ts — 5/5 pass.
• npx tsc --noEmit (project config): only the pre-existing src/routes/users.ts syntax errors present identically on main.
• npx eslint src/routes/health.ts tests/healthIdempotency.test.ts — clean, no errors.