Finish up import script incremental and reliability updates

inventory-server/db/schema.sql

@@ -52,7 +52,7 @@ CREATE TABLE products (
     notifies INT UNSIGNED DEFAULT 0,
     date_last_sold DATE,
     PRIMARY KEY (pid),
-    UNIQUE KEY unique_sku (SKU),
+    INDEX idx_sku (SKU),
     INDEX idx_vendor (vendor),
     INDEX idx_brand (brand),
     INDEX idx_location (location),
@@ -148,7 +148,6 @@ CREATE TABLE purchase_orders (
     received_by INT,
     receiving_history JSON COMMENT 'Array of receiving records with qty, date, cost, receiving_id, and alt_po flag',
     FOREIGN KEY (pid) REFERENCES products(pid),
-    FOREIGN KEY (sku) REFERENCES products(SKU),
     INDEX idx_po_id (po_id),
     INDEX idx_vendor (vendor),
     INDEX idx_status (status),