feat(announcements): add admin/user announcement system

Implements announcements end-to-end (admin CRUD + read status, user list + mark read) with OR-of-AND targeting. Also breaks the ent<->service import cycle by moving schema-facing constants/targeting into a new domain package.

backend/internal/domain/constants.go

@@ -0,0 +1,64 @@
+package domain
+
+// Status constants
+const (
+	StatusActive   = "active"
+	StatusDisabled = "disabled"
+	StatusError    = "error"
+	StatusUnused   = "unused"
+	StatusUsed     = "used"
+	StatusExpired  = "expired"
+)
+
+// Role constants
+const (
+	RoleAdmin = "admin"
+	RoleUser  = "user"
+)
+
+// Platform constants
+const (
+	PlatformAnthropic   = "anthropic"
+	PlatformOpenAI      = "openai"
+	PlatformGemini      = "gemini"
+	PlatformAntigravity = "antigravity"
+)
+
+// Account type constants
+const (
+	AccountTypeOAuth      = "oauth"       // OAuth类型账号（full scope: profile + inference）
+	AccountTypeSetupToken = "setup-token" // Setup Token类型账号（inference only scope）
+	AccountTypeAPIKey     = "apikey"      // API Key类型账号
+)
+
+// Redeem type constants
+const (
+	RedeemTypeBalance      = "balance"
+	RedeemTypeConcurrency  = "concurrency"
+	RedeemTypeSubscription = "subscription"
+)
+
+// PromoCode status constants
+const (
+	PromoCodeStatusActive   = "active"
+	PromoCodeStatusDisabled = "disabled"
+)
+
+// Admin adjustment type constants
+const (
+	AdjustmentTypeAdminBalance     = "admin_balance"     // 管理员调整余额
+	AdjustmentTypeAdminConcurrency = "admin_concurrency" // 管理员调整并发数
+)
+
+// Group subscription type constants
+const (
+	SubscriptionTypeStandard     = "standard"     // 标准计费模式（按余额扣费）
+	SubscriptionTypeSubscription = "subscription" // 订阅模式（按限额控制）
+)
+
+// Subscription status constants
+const (
+	SubscriptionStatusActive    = "active"
+	SubscriptionStatusExpired   = "expired"
+	SubscriptionStatusSuspended = "suspended"
+)