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/users.sql

@@ -15,7 +15,17 @@ WHERE username = $1;
 -- name: ListUsers :many
 SELECT id, username, name, role, avatar_url, state, created_at
 FROM users
-ORDER BY created_at ASC
+WHERE (
+  sqlc.arg(search) = ''
+  OR username ILIKE '%' || sqlc.arg(search) || '%'
+  OR name ILIKE '%' || sqlc.arg(search) || '%'
+)
+AND (
+  sqlc.arg(cursor_created) = ''
+  OR created_at < sqlc.arg(cursor_created)
+  OR (created_at = sqlc.arg(cursor_created) AND id < sqlc.arg(cursor_id))
+)
+ORDER BY created_at DESC, id DESC
 LIMIT sqlc.arg(row_limit);
 
 -- name: CountAdmins :one