feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/handler/admin/account_handler.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/domain"
 	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/claude"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
@@ -1490,3 +1491,9 @@ func (h *AccountHandler) BatchRefreshTier(c *gin.Context) {
 
 	response.Success(c, results)
 }
+
+// GetAntigravityDefaultModelMapping 获取 Antigravity 平台的默认模型映射
+// GET /api/v1/admin/accounts/antigravity/default-model-mapping
+func (h *AccountHandler) GetAntigravityDefaultModelMapping(c *gin.Context) {
+	response.Success(c, domain.DefaultAntigravityModelMapping)
+}