[Chore]: Use of ServerType enum across the codebase

[M0NsTeRRR]

Area

refactor / tech debt

Summary

As discussed with @fallenbagel we should use across the codebase this enum https://github.com/seerr-team/seerr/blob/develop/server/constants/server.ts#L8 and add plex to avoid typo issues.

Reference : #3324 (comment)

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