refactor: replace sync.Map credits state with AICredits rate limit key

Replace process-memory sync.Map + per-model runtime state with a single
"AICredits" key in model_rate_limits, making credits exhaustion fully
isomorphic with model-level rate limiting.

Scheduler: rate-limited accounts with overages enabled + credits available
are now scheduled instead of excluded.

Forwarding: when model is rate-limited + credits available, inject credits
proactively without waiting for a 429 round trip.

Storage: credits exhaustion stored as model_rate_limits["AICredits"] with
5h duration, reusing SetModelRateLimit/isRateLimitActiveForKey.

Frontend: show credits_active (yellow ⚡) when model rate-limited but
credits available, credits_exhausted (red) when AICredits key active.

Tests: add unit tests for shouldMarkCreditsExhausted, injectEnabledCreditTypes,
clearCreditsExhausted, and update existing overages tests.

backend/internal/service/antigravity_quota_scope.go

@@ -32,6 +32,10 @@ func (a *Account) IsSchedulableForModelWithContext(ctx context.Context, requeste
 		return false
 	}
 	if a.isModelRateLimitedWithContext(ctx, requestedModel) {
+		// Antigravity + overages 启用 + 积分未耗尽 → 放行（有积分可用）
+		if a.Platform == PlatformAntigravity && a.IsOveragesEnabled() && !a.isCreditsExhausted() {
+			return true
+		}
 		return false
 	}
 	return true