fix: preserve JSON quotes in Apprise MQTT notifications

[guillochon]

Summary

• Unescape HTML entities (e.g. ") when reading the Apprise notification body so JSON payloads keep real quotes instead of entities.
• Omit the notification title when the body is JSON, because title-less services like MQTT prepend the title and break the payload.
• Harden the settings UI round-trip for the body (html_entity_decode on save/test, htmlspecialchars when rendering the textarea) and add a regression test.

Test plan

• Set Apprise body to a JSON template with quoted fields (e.g. {"Common_Name": "$comname", ...}) and an mqtt:// destination
• Update Settings, then Send Test Notification
• Confirm the MQTT payload is valid JSON with real " characters and no prepended title
• Confirm a normal (non-JSON) text body still includes the title as before
• Run python -m unittest tests.test_apprise_notifications

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