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/handler/dto/announcement.go

@@ -0,0 +1,75 @@
+package dto
+
+import (
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+type Announcement struct {
+	ID      int64  `json:"id"`
+	Title   string `json:"title"`
+	Content string `json:"content"`
+	Status  string `json:"status"`
+
+	Targeting service.AnnouncementTargeting `json:"targeting"`
+
+	StartsAt *time.Time `json:"starts_at,omitempty"`
+	EndsAt   *time.Time `json:"ends_at,omitempty"`
+
+	CreatedBy *int64 `json:"created_by,omitempty"`
+	UpdatedBy *int64 `json:"updated_by,omitempty"`
+
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+type UserAnnouncement struct {
+	ID      int64  `json:"id"`
+	Title   string `json:"title"`
+	Content string `json:"content"`
+
+	StartsAt *time.Time `json:"starts_at,omitempty"`
+	EndsAt   *time.Time `json:"ends_at,omitempty"`
+
+	ReadAt *time.Time `json:"read_at,omitempty"`
+
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+func AnnouncementFromService(a *service.Announcement) *Announcement {
+	if a == nil {
+		return nil
+	}
+	return &Announcement{
+		ID:        a.ID,
+		Title:     a.Title,
+		Content:   a.Content,
+		Status:    a.Status,
+		Targeting: a.Targeting,
+		StartsAt:  a.StartsAt,
+		EndsAt:    a.EndsAt,
+		CreatedBy: a.CreatedBy,
+		UpdatedBy: a.UpdatedBy,
+		CreatedAt: a.CreatedAt,
+		UpdatedAt: a.UpdatedAt,
+	}
+}
+
+func UserAnnouncementFromService(a *service.UserAnnouncement) *UserAnnouncement {
+	if a == nil {
+		return nil
+	}
+	return &UserAnnouncement{
+		ID:        a.Announcement.ID,
+		Title:     a.Announcement.Title,
+		Content:   a.Announcement.Content,
+		StartsAt:  a.Announcement.StartsAt,
+		EndsAt:    a.Announcement.EndsAt,
+		ReadAt:    a.ReadAt,
+		CreatedAt: a.Announcement.CreatedAt,
+		UpdatedAt: a.Announcement.UpdatedAt,
+	}
+}
+