[Task]: Migrate translation status constants to an enum

[hannaseithe]

⚙️ Task

Task Description

What

Migrate the translation status string constants in integreat_cms/cms/constants/translation_status.py to a proper Python enum.

Why

Currently these are plain Final string constants ("stringly typed"). An enum would make states mutually exclusive by type, aid in catching errors when new states are added, and enable exhaustive matching. Raised during review of #4504.

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Task Category

♻️ Tech Debt — Addressing accumulated shortcuts

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Steps / Implementation Plan

• Convert constants in translation_status.py to a StrEnum or similar
• Update all references across the codebase
• Ensure template usage still works (Django templates access enum values differently)

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Acceptance Criteria

• All translation status constants are defined as an enum
• No plain string comparisons remain against these values
• Existing tests pass

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Parent / Related Issues

• Raised during review of Move machine translation processing to Celery (Backend) #4504

[andrew8er]

There are probably a lot of other places, where a string could be replaced with an enum.

Another, less intrusive option would be to use a Literal["val1", "val2", …] type. Both have up and downsides.