api: extract _rowcount(result) helper to dedupe 6x cast(CursorResult[Any], ...)

[countercheck]

Source: codebase audit, 2026-06-11 (audit finding F8, Tier 3)
Severity: Low
Category: Type hygiene / DRY

Problem

The same six-line type dance is repeated to access `.rowcount` on a SQLAlchemy result:

```python
result = await session.execute(stmt)

psycopg's CursorResult exposes .rowcount, but mypy needs the cast

deleted = cast(CursorResult[Any], result).rowcount or 0
```

Sites in api/survey_engine/service.py: 374, 614, 624, 635, 957, 971.

The `or 0` guards against `None`, but the cast suppresses mypy without explaining why `.rowcount` is Optional. Future refactors might drop the guard.

Suggested approach

Add a one-liner helper somewhere (e.g. `api/db_helpers.py` or near the existing imports):

```python
def rowcount(result: Result[Any]) -> int:
return cast(CursorResult[Any], result).rowcount or 0
```

Replace the six call sites with `rowcount(result)`.

Verification

`uv run mypy api/` should remain clean. No behavior change.

Priority

Nice-to-have, not worth its own PR. Fold into a touch of `service.py` for another reason.