fix: cache GitHub API responses for compare endpoint to avoid rate limiting

[Tanisha-sharma7302]

Fixes #2841

Problem

The /api/metrics/compare endpoint had a TTL reserved for it in metrics-cache.ts
(compare: 30 * 60) but never actually used it — instead it relied on a custom
Supabase-based cache keyed per viewer + target user + calendar day. This meant
every new (viewer, target, day) combination triggered 4-5 fresh GitHub API calls
with no short-term caching, contributing to rate-limit (403) responses under
repeated/rapid dashboard usage as described in the issue.

Fix

Switched the compare endpoint to use the existing withMetricsCache-style
Redis/memory caching system (cacheGet/cacheSet from metrics-cache.ts),
using the already-reserved 30-minute TTL — consistent with every other metrics
endpoint in the app. Also respects the existing ?refresh=true /
?bypassCache=true cache-bypass convention.

Testing

• pnpm run type-check passes with no errors
• Existing test suite (pnpm vitest run) unaffected

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