feat: 添加通知微服务，支持站内通知已读状态

desc/sql/notification/notifications.sql

@@ -0,0 +1,24 @@
+CREATE TABLE notifications
+(
+    id         BIGINT PRIMARY KEY,
+    user_id    BIGINT       NOT NULL,
+    type       VARCHAR(20)  NOT NULL,
+    title      VARCHAR(200) NOT NULL,
+    content    TEXT         NOT NULL,
+    read       BOOLEAN      NOT NULL DEFAULT FALSE,
+    link       VARCHAR(500),
+    created_at TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+
+    CONSTRAINT chk_notification_type CHECK (type IN ('order', 'community', 'system'))
+);
+
+CREATE INDEX idx_notifications_user_created ON notifications (user_id, created_at DESC);
+CREATE INDEX idx_notifications_user_read_created ON notifications (user_id, read, created_at DESC);
+CREATE INDEX idx_notifications_user_type_created ON notifications (user_id, type, created_at DESC);
+
+CREATE TRIGGER trigger_notifications_updated_at
+    BEFORE UPDATE
+    ON notifications
+    FOR EACH ROW
+    EXECUTE FUNCTION update_updated_at_column();