Concurrent or retried com.atproto.server.refreshSession calls presenting the
same refresh token hit the reuse-detection path, which deleted the session and
returned "Refresh token has been revoked due to suspected compromise" —
logging users out at random. The legacy flow had no grace period, unlike OAuth.
Mirror the reference atproto PDS: every rotated refresh token gets a 2h grace
window measured from its own rotation time (used_refresh_tokens.used_at in
postgres; a rotated_at_ms field appended to the metastore used-marker, with
old-format markers decoding as outside the window). A refresh presenting a
recently-rotated token is served the session's current tokens, re-minted on
the fly with the same jti/expiry — signed JWTs are never persisted. Reuse
outside the window still revokes the session.
The grace lookup returns the session's encrypted signing key so the handler
verifies the presented token's signature before minting replacement tokens or
revoking a session; a forged token bearing a known jti gets a generic
rejection with no side effects.
Integration tests asserting the old replay-gets-401 behavior are reworked to
the new contract and now also cover forged-signature replays and
out-of-window revocation.
RFC 7519 §5.1 recommends the uppercase "JWT" typ for compatibility with
legacy implementations, and it matches the reference @atproto/pds. Parsing
already lowercases, so existing lowercase "jwt" tokens still verify.
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