feat: Scheduled Maintenance feature complete

backend/src/features/maintenance/migrations/003_enhance_maintenance_schedules.sql

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+-- Add schedule_type to support different scheduling methods
+ALTER TABLE maintenance_schedules
+  ADD COLUMN schedule_type VARCHAR(20) NOT NULL DEFAULT 'interval'
+    CHECK (schedule_type IN ('interval', 'fixed_date', 'time_since_last'));
+
+-- Add fixed_due_date for fixed date schedules
+ALTER TABLE maintenance_schedules
+  ADD COLUMN fixed_due_date DATE;
+
+-- Add reminder columns for progressive notifications
+ALTER TABLE maintenance_schedules
+  ADD COLUMN reminder_days_1 INTEGER
+    CHECK (reminder_days_1 IN (1, 7, 14, 30, 60));
+
+ALTER TABLE maintenance_schedules
+  ADD COLUMN reminder_days_2 INTEGER
+    CHECK (reminder_days_2 IN (1, 7, 14, 30, 60));
+
+ALTER TABLE maintenance_schedules
+  ADD COLUMN reminder_days_3 INTEGER
+    CHECK (reminder_days_3 IN (1, 7, 14, 30, 60));
+
+-- Index for filtering by schedule type
+CREATE INDEX idx_maintenance_schedules_schedule_type ON maintenance_schedules(schedule_type);