Index: docs/paid-posts-design.md
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 `PAID_SECRET` (32 random bytes) lives in env, like the other secrets.
+
+## Concurrency (a property of the cookie-less model)
+
+Because there is no session and no "current user", the model is inherently
+multi-user. Two people unlock side by side with no shared state to collide:
+each request carries its own assertion, verified against that credential's own
+public key, and the content goes back in that one response. `paid_entitlements`
+is keyed per credential, so N passkeys are N independent rows. The challenge is
+stateless (the signed blob), so there is no single "pending challenge" slot a
+second visitor could overwrite. Unlike a cookie session, "two people in the same
+browser" cannot clobber each other. The only caveat is a shared browser profile:
+the passkey picker would then list both passkeys (a small visibility hint, not
+access).
 
 ## Data model (additive)