A batch entry matching a stored workout stays first-write-wins for the
workout row. What it can do now is hand over a heart-rate series the
stored workout does not have yet: the series is attached, the row itself
is not written at all, and the entry reports back as `enriched`. A
workout that already carries a series is a no-op, so a client can walk
its whole history as often as it likes.
- The pre-flight probe now answers both questions at once, whether the
row exists and whether it already has a series, and carries the id so
the attach addresses the stored workout without a second round trip.
- The attach rides the same child write as a fresh workout's series.
RETURNING names the rows that landed, so a workout that gained a
series between probe and write reports as the duplicate it is instead
of claiming an attach that never happened.
- A `samples` array that fails validation is refused per entry with
`reason: "invalid_samples"` rather than failing the batch. One
unusable session out of a hundred must not stall a history sweep the
client cannot repair by retrying; every other validation failure keeps
failing the batch.
- The wide event and the audit row tally enrichments. Counts only, the
series never reaches either.
- `enriched` is the one contract movement: additive on the per-entry
status, request shape unchanged. OpenAPI regenerated.
Integration coverage against a real Postgres pins the attach, the
repeat as a no-op, the refusal of a bad series alongside entries that
land, a mixed batch reporting each status, and the stored workout row
compared whole before and after, `updated_at` included.
