fix(antigravity): fast-fail on proxy unavailable, temp-unschedule account

## Problem

When a proxy is unreachable, token refresh retries up to 4 times with
30s timeout each, causing requests to hang for ~2 minutes before
failing with a generic 502 error. The failed account is not marked,
so subsequent requests keep hitting it.

## Changes

### Proxy connection fast-fail
- Set TCP dial timeout to 5s and TLS handshake timeout to 5s on
  antigravity client, so proxy connectivity issues fail within 5s
  instead of 30s
- Reduce overall HTTP client timeout from 30s to 10s
- Export `IsConnectionError` for service-layer use
- Detect proxy connection errors in `RefreshToken` and return
  immediately with "proxy unavailable" error (no retries)

### Token refresh temp-unschedulable
- Add 8s context timeout for token refresh on request path
- Mark account as temp-unschedulable for 10min when refresh fails
  (both background `TokenRefreshService` and request-path
  `GetAccessToken`)
- Sync temp-unschedulable state to Redis cache for immediate
  scheduler effect
- Inject `TempUnschedCache` into `AntigravityTokenProvider`

### Account failover
- Return `UpstreamFailoverError` on `GetAccessToken` failure in
  `Forward`/`ForwardGemini` to trigger handler-level account switch
  instead of returning 502 directly

### Proxy probe alignment
- Apply same 5s dial/TLS timeout to shared `httpclient` pool
- Reduce proxy probe timeout from 30s to 10s

backend/internal/service/wire.go

@@ -114,11 +114,13 @@ func ProvideAntigravityTokenProvider(
 	tokenCache GeminiTokenCache,
 	antigravityOAuthService *AntigravityOAuthService,
 	refreshAPI *OAuthRefreshAPI,
+	tempUnschedCache TempUnschedCache,
 ) *AntigravityTokenProvider {
 	p := NewAntigravityTokenProvider(accountRepo, tokenCache, antigravityOAuthService)
 	executor := NewAntigravityTokenRefresher(antigravityOAuthService)
 	p.SetRefreshAPI(refreshAPI, executor)
 	p.SetRefreshPolicy(AntigravityProviderRefreshPolicy())
+	p.SetTempUnschedCache(tempUnschedCache)
 	return p
 }