feat(antigravity): progressive penalty for consecutive INTERNAL 500 errors

When an antigravity account returns 500 "Internal error encountered."
on all 3 retry attempts, increment a Redis counter and apply escalating
penalties:
- 1st round: temp unschedulable 10 minutes
- 2nd round: temp unschedulable 10 hours
- 3rd round: permanently mark as error

Counter resets on any successful response (< 400).

backend/internal/repository/wire.go

@@ -81,6 +81,7 @@ var ProviderSet = wire.NewSet(
 	NewAPIKeyCache,
 	NewTempUnschedCache,
 	NewTimeoutCounterCache,
+	NewInternal500CounterCache,
 	ProvideConcurrencyCache,
 	ProvideSessionLimitCache,
 	NewRPMCache,