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/migrations/045_add_announcements.sql

@@ -0,0 +1,44 @@
+-- 创建公告表
+CREATE TABLE IF NOT EXISTS announcements (
+    id BIGSERIAL PRIMARY KEY,
+    title VARCHAR(200) NOT NULL,
+    content TEXT NOT NULL,
+    status VARCHAR(20) NOT NULL DEFAULT 'draft',
+    targeting JSONB NOT NULL DEFAULT '{}'::jsonb,
+    starts_at TIMESTAMPTZ DEFAULT NULL,
+    ends_at TIMESTAMPTZ DEFAULT NULL,
+    created_by BIGINT DEFAULT NULL REFERENCES users(id) ON DELETE SET NULL,
+    updated_by BIGINT DEFAULT NULL REFERENCES users(id) ON DELETE SET NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- 公告已读表
+CREATE TABLE IF NOT EXISTS announcement_reads (
+    id BIGSERIAL PRIMARY KEY,
+    announcement_id BIGINT NOT NULL REFERENCES announcements(id) ON DELETE CASCADE,
+    user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    read_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    UNIQUE(announcement_id, user_id)
+);
+
+-- 索引
+CREATE INDEX IF NOT EXISTS idx_announcements_status ON announcements(status);
+CREATE INDEX IF NOT EXISTS idx_announcements_starts_at ON announcements(starts_at);
+CREATE INDEX IF NOT EXISTS idx_announcements_ends_at ON announcements(ends_at);
+CREATE INDEX IF NOT EXISTS idx_announcements_created_at ON announcements(created_at);
+
+CREATE INDEX IF NOT EXISTS idx_announcement_reads_announcement_id ON announcement_reads(announcement_id);
+CREATE INDEX IF NOT EXISTS idx_announcement_reads_user_id ON announcement_reads(user_id);
+CREATE INDEX IF NOT EXISTS idx_announcement_reads_read_at ON announcement_reads(read_at);
+
+COMMENT ON TABLE announcements IS '系统公告';
+COMMENT ON COLUMN announcements.status IS '状态: draft, active, archived';
+COMMENT ON COLUMN announcements.targeting IS '展示条件（JSON 规则）';
+COMMENT ON COLUMN announcements.starts_at IS '开始展示时间（为空表示立即生效）';
+COMMENT ON COLUMN announcements.ends_at IS '结束展示时间（为空表示永久生效）';
+
+COMMENT ON TABLE announcement_reads IS '公告已读记录';
+COMMENT ON COLUMN announcement_reads.read_at IS '用户首次已读时间';
+