docs: clarify provider-scoped verification claims

[UgwujaGeorge]

Summary

• Clarify that Base Verify tokens are scoped to a provider.
• Document the difference between one claim per provider account and one claim per campaign participant.
• Update the Prisma example to include provider-scoped uniqueness.

Why

Fixes #31. The README previously described deterministic tokens as enforcing one global claim across every provider, even though it also documents that tokens are unique per provider. That wording could lead builders to implement weaker-than-intended campaign claim rules.

Testing

• git diff --check
• Documentation-only change; no application code changed.

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