The `POST /api/issues/batch-update` handler walked every issue ID and
incremented `updated` regardless of whether the iteration carried any
mutation. When the caller's payload had no recognized field in
`updates` — e.g. status placed at the top level instead of nested,
"update" misspelled as singular, or "updates" missing entirely —
the loop ran N no-op UPDATEs (each if-guard skipped, each COALESCE
preserved the existing value) and the response cheerfully reported
`{"updated": N}` while nothing changed. Reporters mistook the
positive count for success and chased a phantom persistence bug.
Detect at the top of the handler whether any known mutation field is
present in the parsed `updates` payload; if none is, short-circuit
with `{"updated": 0}`. The wire shape stays 200 + `{updated}`
so existing callers don't break — only the count becomes truthful.
Tests cover the three caller shapes that hit this path (status at top
level, empty `updates: {}`, misspelled "update") plus a positive
case that locks in happy-path persistence and counting.
Closes#1660.
