Document shared Cloudflare build variables

[nicu-chiciuc]

Why

The Cloudflare guide recommended one Convex deploy key per API trigger, but Samebase intentionally
mirrors Cloudflare's current dashboard model. The dashboard shows one build-variable table, and
both keys must stay on both triggers so both remain visible.

Summary

• Tell dashboard and API users to store both Convex deploy keys on both Workers Builds triggers.
• Record that the shared layout stays in place until Cloudflare exposes separate production and
  preview build-variable views.
• Preserve the existing branch-based selection of the key that Convex receives.

Validation

• vp check
• vp test (23 tests)

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