chore(main): release 3.1.0

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🤖 I have created a release beep boop

3.1.0 (2026-08-11)

Features

• optional MQTT username/password with portal restyling (9b0ab7a)

Bug Fixes

• code review findings — dangling reference, printf, trailing return type (c6c3bc4)
• cpplint header guard style in Version.h (SRC_VERSION_H_) (685f6e1)
• license: add SPDX-License-Identifier to all source files (61135d6)

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Super-linter summary

Language	Validation result
CPP	Fail ❌
EDITORCONFIG	Pass ✅
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CPP

Done processing /github/workspace/src/Version.h
Total errors found: 2/github/workspace/src/Version.h:7:  #ifndef header guard has wrong style, please use: SRC_VERSION_H_  [build/header_guard] [5]
/github/workspace/src/Version.h:14:  #endif line should be "#endif  // SRC_VERSION_H_"  [build/header_guard] [5]

MARKDOWN

/github/workspace/CHANGELOG.md:69 error MD024/no-duplicate-heading Multiple headings with the same content [Context: "Features"]

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🤖 Release is at https://github.com/smart-swimmingpool/monitor/releases/tag/v3.1.0 🌻