Add categories table and update/optimize import script

inventory-server/db/schema.sql

@@ -78,4 +78,23 @@ CREATE TABLE IF NOT EXISTS purchase_orders (
     INDEX idx_vendor (vendor),
     INDEX idx_status (status),
     UNIQUE KEY unique_po_product (po_id, product_id)
+);
+
+-- Create categories table
+CREATE TABLE IF NOT EXISTS categories (
+    id BIGINT AUTO_INCREMENT PRIMARY KEY,
+    name VARCHAR(100) NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE KEY unique_name (name)
+);
+
+-- Create product_categories junction table
+CREATE TABLE IF NOT EXISTS product_categories (
+    product_id BIGINT NOT NULL,
+    category_id BIGINT NOT NULL,
+    PRIMARY KEY (product_id, category_id),
+    FOREIGN KEY (product_id) REFERENCES products(product_id) ON DELETE CASCADE,
+    FOREIGN KEY (category_id) REFERENCES categories(id) ON DELETE CASCADE,
+    INDEX idx_category (category_id),
+    INDEX idx_product (product_id)
 );