fix: unescape HTML entities in plain-text email bodies

[okxint]

django.utils.html.strip_tags() strips HTML markup but leaves character entities (&, <, etc.) in the output as literals. In invitation and authentication emails, URLs are rendered inside HTML anchor tags — so & in query strings becomes & in the HTML source. After stripping tags the entity survives, producing fallback links like:

https://app.plane.so/workspace-invitations/?invitation_id=…&slug=…

Email clients that display the plain-text part pass this literally to the browser, which sees amp;slug and amp;token instead of slug and token. The invitation page never fetches /api/workspaces/<slug>/invitations/<id>/ and stays on the loading screen indefinitely.

Wrapping the result in html.unescape() decodes all entities before the text is stored or sent. Applied consistently across every email background task that follows the same pattern.

Fixes #9497

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