feat: optimize MODEL_CAPACITY_EXHAUSTED retry and remove extra failover retries

- MODEL_CAPACITY_EXHAUSTED now uses independent retry strategy:
  - retryDelay < 20s: wait actual retryDelay then retry once
  - retryDelay >= 20s or missing: retry up to 5 times at 20s intervals
  - Still capacity exhausted after retries: switch account (failover)
  - Different error during retry (e.g. 429): handle by actual error code
  - No model rate limit set (capacity != rate limit)

- Remove Antigravity extra failover retries feature:
  Same-account retry mechanism (cherry-picked) makes it redundant.
  Removed: antigravityExtraRetries config, sleepFixedDelay, skip-non-antigravity logic.

backend/internal/config/config.go

@@ -279,9 +279,6 @@ type GatewayConfig struct {
 	// Antigravity 429 fallback 限流时间（分钟），解析重置时间失败时使用
 	AntigravityFallbackCooldownMinutes int `mapstructure:"antigravity_fallback_cooldown_minutes"`
 
-	// 默认重试用完后，额外使用 Antigravity 账号重试的最大次数（0 表示禁用）
-	AntigravityExtraRetries int `mapstructure:"antigravity_extra_retries"`
-
 	// Scheduling: 账号调度相关配置
 	Scheduling GatewaySchedulingConfig `mapstructure:"scheduling"`