feat(?): ⭐ Discord ID as unique identifier for users

[Nudelsuppe42]

[1] Feature Description
In general, a Discord ID is unique, but right now the DB schema does not think it is (there may be duplicated entries right now).
Converting the discordId to be unique would allow to query users by it and link users to e.g. claims just by their discord Id without the need to fetch users by other means (which involves an additional request).

[2] Screenshots
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[3] Scope

• DB Schema
• Claim endpoints with owner or builder connect (also team public endpoints!)
• possibly the search user endpoints

[4] Additional context
-/-