Two callbacks with the same state could both exchange the authorization code upstream (concurrently or by replay); the session is now atomically claimed under the proxy lock before the exchange.
GET /posters/ and the /auth/result long-poll had no per-IP limiting or concurrency bound, and every poster lookup serialized through an exclusive store lock; both endpoints now use the established limiter and non-expired hits take a read lock. The limiter also tracks active transfers per IP with caps below the global limits (4 for fetches, 2 for uploads), because one unauthenticated client could otherwise take all 16 global poster-fetch slots and hold them through slow ServeContent reads, starving everyone else with 429s; concurrency checks precede bucket charges so a capped denial consumes no admission tokens.
/auth/result gets its own per-IP budget instead of sharing /auth/start's burst-3 bucket, where two concurrent NAT'd sign-ins 429'd on the fourth request. It is charged only after the poll secret validates (bogus requests keep the generic 410 and cost nothing), denials carry an honest Retry-After, and the Dart poller - which treated 429 as terminal and abandoned a valid session - retries them until the session's 10-minute lifetime expires.
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