Parallelize health checks for all providers

[GoDiao]

checkAllProvidersHealth() in src/health.ts currently checks providers one by one in a for loop. This is slow when you have many providers configured. It should run checks concurrently using Promise.all.

Files to change:

• src/health.ts — replace the sequential for loop with Promise.all

Acceptance criteria:

• Health checks run in parallel, reducing total check time
• Individual check failures still handled gracefully (use Promise.allSettled or catch per-provider)

Note: Be careful to keep error handling per-provider so one failed check doesn't block others.