Add free tier rate limiting with Upstash Redis and API key handling

lib/rate-limit.ts

@@ -0,0 +1,15 @@
+import { Ratelimit } from "@upstash/ratelimit"
+import { Redis } from "@upstash/redis"
+
+const redis = new Redis({
+  url: process.env.UPSTASH_REDIS_REST_URL!,
+  token: process.env.UPSTASH_REDIS_REST_TOKEN!,
+})
+
+// Free tier: 1 comic per week (fixed window)
+export const freeTierRateLimit = new Ratelimit({
+  redis,
+  limiter: Ratelimit.fixedWindow(1, "7 d"),
+  analytics: true,
+  prefix: "ratelimit:free-comics",
+})