feat: Scheduled Maintenance feature complete

backend/src/features/notifications/migrations/003_create_user_notifications.sql

@@ -0,0 +1,19 @@
+-- user_notifications: In-app notification center for users
+CREATE TABLE user_notifications (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  user_id VARCHAR(255) NOT NULL,
+  notification_type VARCHAR(50) NOT NULL,
+  title VARCHAR(255) NOT NULL,
+  message TEXT NOT NULL,
+  reference_type VARCHAR(50),
+  reference_id UUID,
+  vehicle_id UUID,
+  is_read BOOLEAN DEFAULT false,
+  created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+  read_at TIMESTAMP WITH TIME ZONE
+);
+
+-- Indexes for performance
+CREATE INDEX idx_user_notifications_user_id ON user_notifications(user_id);
+CREATE INDEX idx_user_notifications_created_at ON user_notifications(created_at DESC);
+CREATE INDEX idx_user_notifications_unread ON user_notifications(user_id, created_at DESC) WHERE is_read = false;

backend/src/features/notifications/migrations/004_create_sent_notification_tracker.sql

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+-- sent_notification_tracker: Prevent duplicate notifications for schedules
+CREATE TABLE sent_notification_tracker (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  schedule_id UUID NOT NULL,
+  notification_date DATE NOT NULL,
+  reminder_level INTEGER NOT NULL CHECK (reminder_level IN (1, 2, 3)),
+  delivery_method VARCHAR(20) NOT NULL CHECK (delivery_method IN ('email', 'in_app', 'both')),
+  created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+
+  CONSTRAINT fk_sent_notification_schedule
+    FOREIGN KEY (schedule_id)
+    REFERENCES maintenance_schedules(id)
+    ON DELETE CASCADE,
+
+  CONSTRAINT unique_notification_per_schedule_date_level
+    UNIQUE (schedule_id, notification_date, reminder_level)
+);
+
+-- Indexes for performance
+CREATE INDEX idx_sent_notification_tracker_schedule_id ON sent_notification_tracker(schedule_id);
+CREATE INDEX idx_sent_notification_tracker_notification_date ON sent_notification_tracker(notification_date);