def _validate_value(key: str, value) -> tuple:
    d = SETTING_DEFS.get(key)
    if not d:
        return False, f"Unknown setting key: {key}"
    t = d["type"]
    try:
        if t == "float":
            v = float(value)
            if "min" in d and v < d["min"]:
                return False, f"Value below minimum {d['min']}"
            if "max" in d and v > d["max"]:
                return False, f"Value above maximum {d['max']}"
            return True, v
        if t == "int":
            v = int(value)
            if "min" in d and v < d["min"]:
                return False, f"Value below minimum {d['min']}"
            if "max" in d and v > d["max"]:
                return False, f"Value above maximum {d['max']}"
            return True, v
        if t == "bool":
            if isinstance(value, str):
                v = value.strip().lower() in ("1", "true", "yes", "on")
            else:
                v = bool(value)
            return True, v
        if t == "str":
            v = str(value).strip()
            if "max_len" in d and len(v) > d["max_len"]:
                return False, f"Max length {d['max_len']}"
            if key == "attachment_kind" and v not in ("invoice", "order", "receipt", "document"):
                return False, "attachment_kind must be invoice|order|receipt|document"
            if key == "invoice_format" and ("{" not in v and v):
                return False, "Format must contain tokens like {prefix}{R#}{seq4}"
            return True, v
    except (TypeError, ValueError):
        return False, f"Value must be {t}"
    return False, "Invalid value"


@router.get("/list")
===EXPORT===
async def export_logs(campaign_id: int = Query(0),
                      outcome: str = Query(""),
                      db: AsyncSession = Depends(get_db),
                      user: User = Depends(get_current_user)):
    """CSV export of send attempts for a campaign (all pages)."""
    q = (select(SendAttempt)
         .join(Campaign, Campaign.id == SendAttempt.campaign_id)
         .where(Campaign.user_id == user.id))
    if campaign_id:
        q = q.where(SendAttempt.campaign_id == campaign_id)
    if outcome:
        q = q.where(SendAttempt.outcome == outcome)
    rows = (await db.execute(q.order_by(SendAttempt.id.asc()))).scalars().all()

    def generate():
        buf = io.StringIO()
        w = csv.writer(buf)
        w.writerow(["attempt_id", "campaign_id", "email", "attempt_number", "outcome",
                    "error_class", "error_message", "message_id", "method",
                    "provider_response", "timestamp"])
        yield buf.getvalue()
        buf.seek(0); buf.truncate(0)
        for a in rows:
            w.writerow([a.id, a.campaign_id, a.recipient_email, (a.retry_no or 0) + 1, a.outcome,
                        a.error_class, (a.failure_reason or "")[:500], a.message_id, a.method,
                        (a.provider_response or "")[:500],
                        a.created_at.isoformat() if a.created_at else ""])
            yield buf.getvalue()
            buf.seek(0); buf.truncate(0)

    return StreamingResponse(generate(), media_type="text/csv",
                             headers={"Content-Disposition":
                                      f"attachment; filename=send_logs{('_' + str(campaign_id)) if campaign_id else 's'}.csv"})