CVE-2026-64865: New API: Redis user quota cache overwrite via PUT /api/user/self allows quota bypass

Authenticated users can repeatedly call PUT /api/user/self with language or sidebar_modules while relay requests are consuming quota. The settings path reads a full User snapshot and writes it back through User.Update(), which refreshes Redis with RedisHSetObj and overwrites the Quota field. This can erase concurrent HINCRBY quota deductions and keep cached balance artificially high, allowing calls far beyond the paid quota.