Address production-readiness review: clearer errors, safer votes, and ops hardening.

Distinguish auth/lookup failures, make votes idempotent on visible questions, bound shutdown, page admin users, LRU throttle, trusted-proxy CIDRs, avatar cleanup, versioned migrations, and session cleanup logging.

db/queries/sessions.sql

@@ -13,6 +13,6 @@ SET data = excluded.data, expiry = excluded.expiry;
 DELETE FROM sessions
 WHERE token = $1;
 
--- name: DeleteExpiredSessions :exec
+-- name: DeleteExpiredSessions :execrows
 DELETE FROM sessions
 WHERE expiry <= now();