feat(redis): add configurable key namespace prefix (#11042)

Validated on the combined board over tip 80d931ae: quota-redis-store (incl. the KEY_PREFIX derivation test), local-redis-status and rate-limiter-redis-optional green, typecheck:core clean. One pre-merge fix pushed to the branch: docs/reference/ENVIRONMENT.md gained the REDIS_KEY_PREFIX row (env-doc-sync gate requires every .env.example var documented). Board note: the redis tests leave an ioredis retry handle open and hang the runner exit locally — assertions all pass; pre-existing pattern, not from this PR. Thank you @MeRezaRezaei!

src/lib/quota/redisQuotaStore.ts

@@ -72,7 +72,7 @@ export function resetRedisClient(): void {
 // Key helpers
 // ---------------------------------------------------------------------------
 
-const KEY_PREFIX = "omniroute:quota";
+const KEY_PREFIX = `${process.env.REDIS_KEY_PREFIX?.trim() || "omniroute:"}quota`;
 
 function bucketKey(apiKeyId: string, dimensionKey: string, bucketIndex: number): string {
   return `${KEY_PREFIX}:${apiKeyId}:${dimensionKey}:${bucketIndex}`;