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/domain/constants.go

@@ -64,3 +64,38 @@ const (
 	SubscriptionStatusExpired   = "expired"
 	SubscriptionStatusSuspended = "suspended"
 )
+
+// DefaultAntigravityModelMapping 是 Antigravity 平台的默认模型映射
+// 当账号未配置 model_mapping 时使用此默认值
+// 与前端 useModelWhitelist.ts 中的 antigravityDefaultMappings 保持一致
+var DefaultAntigravityModelMapping = map[string]string{
+	// Claude 白名单
+	"claude-opus-4-6-thinking":   "claude-opus-4-6-thinking", // 官方模型
+	"claude-opus-4-6":            "claude-opus-4-6-thinking", // 简称映射
+	"claude-opus-4-5-thinking":   "claude-opus-4-6-thinking", // 迁移旧模型
+	"claude-sonnet-4-5":          "claude-sonnet-4-5",
+	"claude-sonnet-4-5-thinking": "claude-sonnet-4-5-thinking",
+	// Claude 详细版本 ID 映射
+	"claude-opus-4-5-20251101":   "claude-opus-4-6-thinking", // 迁移旧模型
+	"claude-sonnet-4-5-20250929": "claude-sonnet-4-5",
+	// Claude Haiku → Sonnet（无 Haiku 支持）
+	"claude-haiku-4-5":          "claude-sonnet-4-5",
+	"claude-haiku-4-5-20251001": "claude-sonnet-4-5",
+	// Gemini 2.5 白名单
+	"gemini-2.5-flash":          "gemini-2.5-flash",
+	"gemini-2.5-flash-lite":     "gemini-2.5-flash-lite",
+	"gemini-2.5-flash-thinking": "gemini-2.5-flash-thinking",
+	"gemini-2.5-pro":            "gemini-2.5-pro",
+	// Gemini 3 白名单
+	"gemini-3-flash":     "gemini-3-flash",
+	"gemini-3-pro-high":  "gemini-3-pro-high",
+	"gemini-3-pro-low":   "gemini-3-pro-low",
+	"gemini-3-pro-image": "gemini-3-pro-image",
+	// Gemini 3 preview 映射
+	"gemini-3-flash-preview":     "gemini-3-flash",
+	"gemini-3-pro-preview":       "gemini-3-pro-high",
+	"gemini-3-pro-image-preview": "gemini-3-pro-image",
+	// 其他官方模型
+	"gpt-oss-120b-medium":    "gpt-oss-120b-medium",
+	"tab_flash_lite_preview": "tab_flash_lite_preview",
+}