Add supply purchase order workflow

backend/db.js

@@ -490,6 +490,33 @@ const initDB = async () => {
             );
         `);
 
+        await pool.query(`
+            CREATE TABLE IF NOT EXISTS supply_purchase_orders (
+                id SERIAL PRIMARY KEY,
+                number VARCHAR(40) UNIQUE,
+                supplier TEXT NOT NULL,
+                status VARCHAR(30) NOT NULL DEFAULT 'draft',
+                expected_date DATE,
+                notes TEXT,
+                created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP,
+                updated_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
+            );
+        `);
+
+        await pool.query(`
+            CREATE TABLE IF NOT EXISTS supply_purchase_order_items (
+                id SERIAL PRIMARY KEY,
+                purchase_order_id INTEGER NOT NULL REFERENCES supply_purchase_orders(id) ON DELETE CASCADE,
+                material TEXT NOT NULL,
+                quantity NUMERIC(14, 4) NOT NULL CHECK (quantity > 0),
+                unit VARCHAR(30) NOT NULL DEFAULT 'kg',
+                unit_cost NUMERIC(14, 4),
+                created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
+            );
+        `);
+
+        await pool.query(`CREATE INDEX IF NOT EXISTS idx_supply_purchase_orders_status ON supply_purchase_orders (status, created_at DESC);`);
+
         await pool.query(`
             ALTER TABLE production_orders
             ADD COLUMN IF NOT EXISTS notes TEXT,