feat(health): background health check for local provider_nodes

Local inference backends (oMLX, Ollama, LM Studio) configured as
provider_nodes have no health monitoring. When a local provider is
down, OmniRoute waits the full timeout before failing.

This adds a background health check that polls local provider_nodes:
- GET /models with 5s timeout for each local node (localhost only)
- In-memory health cache (no DB migration needed)
- Promise.allSettled for parallel checks (one slow node doesn't block)
- Exponential backoff on failures: 30s → 60s → 120s → 300s max
- Reset to 30s on first success after failure
- State transition logging (healthy ↔ unhealthy)
- Expose health status via GET /api/monitoring/health (localProviders)
- Auto-init on first import (same pattern as tokenHealthCheck)
- 401 treated as healthy (server up, auth required)
- isNodeHealthy() returns true if never checked (optimistic default)

src/app/api/monitoring/health/route.ts

@@ -18,6 +18,7 @@ export async function GET() {
     const circuitBreakers = getAllCircuitBreakerStatuses();
     const rateLimitStatus = getAllRateLimitStatus();
     const lockouts = getAllModelLockouts();
+    const { getAllHealthStatuses } = await import("@/lib/localHealthCheck");
 
     // System info
     const system = {
@@ -46,6 +47,7 @@ export async function GET() {
       timestamp: new Date().toISOString(),
       system,
       providerHealth,
+      localProviders: getAllHealthStatuses(),
       rateLimitStatus,
       lockouts,
       setupComplete: settings?.setupComplete || false,