feat(backend): add V16 migration for notifications table and user preference columns

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V16__notifications_and_preferences.sql

@@ -0,0 +1,16 @@
+-- Notification preferences on the user record — no separate entity needed
+ALTER TABLE users ADD COLUMN notify_on_reply   BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE users ADD COLUMN notify_on_mention BOOLEAN NOT NULL DEFAULT false;
+
+-- In-app notifications
+CREATE TABLE notifications (
+    id            UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    recipient_id  UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    type          VARCHAR(32) NOT NULL,  -- 'REPLY' | 'MENTION'
+    document_id   UUID,
+    reference_id  UUID,                  -- commentId that triggered this notification
+    read          BOOLEAN NOT NULL DEFAULT false,
+    created_at    TIMESTAMP NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_notifications_recipient ON notifications(recipient_id, read, created_at DESC);