fix: drop redundant primarykey flags

[mplemay]

Summary

• remove the unique=True and index=True arguments from PrimaryKeyMixin.id so it’s declared as a plain primary key with the existing UUID defaults and init flag preserved
• update the mixin doc comment to describe only the client/server defaults, no explicit unique/index claim
• tighten the mixin test expectations to what remains guaranteed (primary key, server default, init exclusion, client UUID default)

Testing

• Not run (not requested)

for the pull request title use an all-lowercase conventional commit style message (ex: "feat: move dashboard into home route")