[BUG] Handle failed fetch() calls more safely #3121

Bug Description

Right now, if any internal fetch() rejects due to a network/runtime error, the whole route fails after the rate limit has already been consumed.

You currently handle res.ok === false, but not thrown fetch errors or invalid JSON.

Steps to Reproduce

const [contributionsRaw, prsRaw, streakRaw, reposRaw] = await Promise.all([
fetchJsonOrEmpty(
${baseUrl}/api/metrics/contributions?days=90,
headers
),
fetchJsonOrEmpty(
${baseUrl}/api/metrics/prs,
headers
),
fetchJsonOrEmpty(
${baseUrl}/api/metrics/streak,
headers
),
fetchJsonOrEmpty(
${baseUrl}/api/metrics/repos?days=90,
headers
),
]);

This prevents one broken metrics endpoint from killing the entire insight route.

Affected Area

API Routes

Screenshots

No response

Browser & OS

No response

Environment

None

Additional Context

No response