feat(db): add notification inbox tables

- Added `notifications` table to store notification messages.
- Added `readed_notifications` table to track which notifications have been read by users.

ee/tabby-db/migrations/0039_add-notification-inbox.down.sql

@@ -0,0 +1 @@
+-- Add down migration script here

ee/tabby-db/migrations/0039_add-notification-inbox.up.sql

@@ -0,0 +1,20 @@
+CREATE TABLE notifications (
+  id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+
+  created_at TIMESTAMP NOT NULL DEFAULT(DATETIME('now')),
+  updated_at TIMESTAMP NOT NULL DEFAULT(DATETIME('now')),
+
+  -- enum of ADMIN, ALL_USERS
+  kind: TEXT NOT NULL,
+
+  message: TEXT NOT NULL,
+)
+
+CREATE TABLE readed_notifications (
+  id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  notification_id INTEGER NOT NULL,
+
+  FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY (notification_id) REFERENCES notifications(id) ON DELETE CASCADE,
+)