Research Vendel SMS gateway for homelab alerts

## Context
Vendel is an open-source SMS gateway platform that lets you run a full-stack device-based SMS service (PocketBase backend, React/TanStack frontend, Docker + Go tooling).

## Why it matters
A self-hosted SMS gateway could give Bear Stone a more reliable way to send/receive alerts through USB modems or Android devices, and Vendel already includes quota management, webhook hooks, and user/device APIs that might play nicely with Home Assistant automations.

## Proposed next step
Stand up the provided Docker Compose stack, register an Android phone or modem, exercise the webhook path, and verify whether Vendel can replace or augment the existing alert delivery stack touches.

## Research links
- https://github.com/JimScope/vendel

Start with Vendel's provided Docker Compose stack and register an Android phone or modem. Exercise the webhook path and compare its quota, device, user, and webhook capabilities with Bear Stone's existing alert delivery stack. Done means documenting whether Vendel can replace or augment the current setup, including any integration blockers.

Written by the indexing model from the issue text.