Closes #5360

Summary

Audit: docs/issues/audit-export-malformed-json.md (Severity: High).

routes/audit.rs:385 had:

let _ = serde_json::to_writer(&mut buf, &value);

If to_writer failed, the chunk contained only , — producing
output like [ , {...}, , {...} ] which is invalid JSON.

serde_json::to_writer into a Vec<u8> is practically infallible
today (no I/O involved), but the let _ = masked the invariant:
any future refactor that swaps the writer for a real
tokio::io::AsyncWrite (S3 streaming, gzip pipe, network tee, …)
would silently corrupt every export.

Fix

-let _ = serde_json::to_writer(&mut buf, &value);
+serde_json::to_writer(&mut buf, &value)
+    .expect("serializing serde_json::Value into Vec<u8> is infallible");

Makes the infallibility load-bearing — any future writer-swap
that CAN fail is forced to handle the error rather than inherit
the silent-corruption behaviour.

Regression guard

test_stream_json_multi_entry_is_well_formed builds a 5-entry
export and round-trips it through serde_json::from_str::<Vec< serde_json::Value>>. The existing
test_stream_json_includes_prev_hash exercised the 1-entry
case; the new test pins the multi-entry separator handling that
the original let _ = could corrupt under failure.

Verification

cargo test -p librefang-api --lib audit::tests::test_stream_json
cargo clippy -p librefang-api --all-targets -- -D warnings

Both green.

Test plan

• CI green
• GET /api/audit/export?format=json with 10+ audit rows →
  body parses as Vec<Value> (browser dev tools / jq .)