Add Hot Spring Connected Spa integration

Unofficial cloud integration for Watkins/Hot Spring "Connected Spa" hot tubs.
Reverse-engineered from the official Android app.

- climate entity (setpoint + current water temp) via heater/control
- sensors: water temperature, salt output level, salt cartridge
- REST (JWT) login + spa discovery; mTLS fetch of rotating HiveMQ broker creds;
  MQTT subscribe (telemetry) / publish (control)
- config flow prompts for account email + password (stored encrypted, never in repo)
- no secrets committed: vendor mTLS client cert is loaded from host PEM files
  (see README); .gitignore blocks pem/p12/der/key/env

Fixes bug-41mqxddz7zeh

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F6B8b3iYNv6QUftK2FDfYb

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 fritzlab
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.