refactor: replace scope-level rate limiting with model-level rate limiting

Merge functional changes from develop branch:
- Remove AntigravityQuotaScope system (claude/gemini_text/gemini_image)
- Replace with per-model rate limiting using resolveAntigravityModelKey
- Remove model load statistics (IncrModelCallCount/GetModelLoadBatch)
- Simplify account selection to unified priority→load→LRU algorithm
- Remove SetAntigravityQuotaScopeLimit from AccountRepository
- Clean up scope-related UI indicators and API fields

backend/internal/handler/dto/types.go

@@ -2,11 +2,6 @@ package dto
 
 import "time"
 
-type ScopeRateLimitInfo struct {
-	ResetAt      time.Time `json:"reset_at"`
-	RemainingSec int64     `json:"remaining_sec"`
-}
-
 type User struct {
 	ID            int64     `json:"id"`
 	Email         string    `json:"email"`
@@ -129,9 +124,6 @@ type Account struct {
 	RateLimitResetAt *time.Time `json:"rate_limit_reset_at"`
 	OverloadUntil    *time.Time `json:"overload_until"`
 
-	// Antigravity scope 级限流状态（从 extra 提取）
-	ScopeRateLimits map[string]ScopeRateLimitInfo `json:"scope_rate_limits,omitempty"`
-
 	TempUnschedulableUntil  *time.Time `json:"temp_unschedulable_until"`
 	TempUnschedulableReason string     `json:"temp_unschedulable_reason"`