feat(03-01): make quote_items.service_id nullable and add custom_label column

- Remove .notNull() from service_id to allow free-form items without catalog ref
- Add custom_label: text("custom_label") for free-form item label storage
- TypeScript compiles with zero errors (QuoteItem.service_id now string | null)

src/db/schema.ts

@@ -164,11 +164,11 @@ export const quote_items = pgTable("quote_items", {
     .notNull()
     .references(() => clients.id, { onDelete: "cascade" }),
   service_id: text("service_id")
-    .notNull()
-    .references(() => service_catalog.id, { onDelete: "restrict" }),
+    .references(() => service_catalog.id, { onDelete: "restrict" }),  // nullable — free-form items have no catalog ref
   quantity: numeric("quantity", { precision: 10, scale: 2 }).notNull(),
   unit_price: numeric("unit_price", { precision: 10, scale: 2 }).notNull(), // snapshot at time of quote
   subtotal: numeric("subtotal", { precision: 10, scale: 2 }).notNull(),
+  custom_label: text("custom_label"),                                       // free-form item label (when service_id is null)
 });
 
 // ============ RELATIONS ============