⚡ Performance: Implement caching for AnalyticsService

[AJFrio]

💡 What:

• Implemented read-through caching in AnalyticsService.js using Cloudflare KV.
• Aggregated analytics data is now cached for 5 minutes (300 seconds).
• The period parameter is validated against allowed values ('1d', '7d', '30d', '90d', '1y') before generating the cache key to prevent cache poisoning.
• Updated src/routes/admin/analytics.js to inject the KV namespace into AnalyticsService.
• Added a performance benchmark test tests/services/AnalyticsService.perf.test.js.

🎯 Why:

• The previous implementation fetched all payment intents from Stripe on every request, which is slow and resource-intensive.
• Caching significantly reduces latency for repeated requests and lowers the load on the Stripe API.
• Input sanitization ensures the cache cannot be exhausted by malicious requests with random keys.

📊 Measured Improvement:

• Baseline (Uncached): ~76ms per call (simulated latency).
• Optimized (Cached): ~0.05ms for subsequent calls (cache hit).
• Stripe API Calls: Reduced from 1 call per request to 1 call per 5 minutes per period type.

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PR created automatically by Jules for task 5466442581751998397 started by @AJFrio

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