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)
+}

backend/internal/handler/admin/ops_realtime_handler.go

@@ -63,6 +63,43 @@ func (h *OpsHandler) GetConcurrencyStats(c *gin.Context) {
 	response.Success(c, payload)
 }
 
+// GetUserConcurrencyStats returns real-time concurrency usage for all active users.
+// GET /api/v1/admin/ops/user-concurrency
+func (h *OpsHandler) GetUserConcurrencyStats(c *gin.Context) {
+	if h.opsService == nil {
+		response.Error(c, http.StatusServiceUnavailable, "Ops service not available")
+		return
+	}
+	if err := h.opsService.RequireMonitoringEnabled(c.Request.Context()); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	if !h.opsService.IsRealtimeMonitoringEnabled(c.Request.Context()) {
+		response.Success(c, gin.H{
+			"enabled":   false,
+			"user":      map[int64]*service.UserConcurrencyInfo{},
+			"timestamp": time.Now().UTC(),
+		})
+		return
+	}
+
+	users, collectedAt, err := h.opsService.GetUserConcurrencyStats(c.Request.Context())
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	payload := gin.H{
+		"enabled": true,
+		"user":    users,
+	}
+	if collectedAt != nil {
+		payload["timestamp"] = collectedAt.UTC()
+	}
+	response.Success(c, payload)
+}
+
 // GetAccountAvailability returns account availability statistics.
 // GET /api/v1/admin/ops/account-availability
 //