CustomRTServer: async default (SendReceiptsSync=false) risks fiscal-document loss on stateless/cloud hosts

## Problem

`CustomRTServer` defaults `SendReceiptsSync=false` (**async**). In async mode it buffers pending commercial documents to a local disk cache that falls back to `SpecialFolder.Personal` when no `ServiceFolder`/`CacheDirectory` is configured. On **CloudCashbox** (stateless, no persistent volume; `SpecialFolder.Personal` resolves to `/root` — writable but ephemeral), those buffered documents are **lost on pod restart/recycle**.

This is live **today** unless every cloud CustomRTServer cashbox config explicitly sets `sendreceiptssync=true`. Same bug class as the EpsonRTServer incident, on a shipped SCU.

## Evidence

- Default: `CustomRTServerConfiguration.SendReceiptsSync` = `false` — `scu-it/src/fiskaltrust.Middleware.SCU.IT.CustomRTServer/CustomRTServerConfiguration.cs:11`
- `SpecialFolder.Personal` fallback: `scu-it/src/fiskaltrust.Middleware.SCU.IT.CustomRTServer/CustomRTServerCommunicationQueue.cs:36-55`
- Hosted in the same factory as EpsonRTServer: `service-cloudcashbox` `SingletonIITSSCDClientFactory`

## Why NOT to blanket-copy the EpsonRTServer fix

The EpsonRTServer fix (middleware #716 + service-cloudcashbox #132) forces sync on cloud and requires an explicit durable folder for async. Do **not** copy it verbatim to CustomRTServer:

- On **Android**, CustomRTServer's `Personal` fallback is **load-bearing durable storage** (no launcher-injected `servicefolder`, unlike the DE providers). Removing it would break async there.
- Forcing `SendReceiptsSync=true` for **live** CustomRTServer cloud cashboxes changes latency/availability semantics → needs product sign-off.

## Suggested actions

1. Audit production cloud CustomRTServer cashbox configs for `sendreceiptssync`.
2. If forcing sync on cloud, mirror #132's host-side `SendReceiptsSync=true` injection **scoped to CustomRTServer** (after product sign-off).
3. Ensure a durable `servicefolder` is injected for CustomRTServer on **Android** before touching its `Personal` fallback.

Refs: middleware #716, service-cloudcashbox #132 (the EpsonRTServer equivalents).

Start with scu-it/src/fiskaltrust.Middleware.SCU.IT.CustomRTServer/CustomRTServerConfiguration.cs and CustomRTServerCommunicationQueue.cs to confirm the async default and Personal-folder fallback. Audit production CloudCashbox configurations for sendreceiptssync, then compare middleware #716 and service-cloudcashbox #132. Done means an agreed, scoped cloud/Android approach that avoids ephemeral storage loss without breaking Android durability.

Written by the indexing model from the issue text.