Initial implementation of 3D Print Demand plugin

Dashboard plugin for InvenTree showing aggregated demand for 3D printed
parts across open build and sales orders, with colour-coded deficit column.

README.md

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+# InvenTree 3D Print Demand Plugin
+
+Dashboard plugin for InvenTree that shows aggregated demand for 3D printed parts across all open build orders and sales orders.
+
+## Features
+
+- Dashboard panel showing all parts from a configured category
+- Aggregates stock, allocations, and demand from build and sales orders
+- Colour-coded deficit column (red = shortage, green = surplus)
+- Configurable part category with optional subcategory inclusion
+
+## Installation
+
+```bash
+cd Projects/02-engineering/inventree/inventree-print-demand
+pip install -e .
+```
+
+Then restart InvenTree and enable the plugin in **Admin > Plugins**.
+
+## Configuration
+
+In plugin settings, set:
+
+- **Part Category** - select the category containing your 3D printed parts
+- **Include Subcategories** - whether to include parts from subcategories (default: true)
+
+## API
+
+`GET /plugin/print-demand/api/demand/` returns a JSON array of parts sorted by deficit:
+
+```json
+[
+  {
+    "pk": 123,
+    "name": "Housing Top",
+    "IPN": "MP-001",
+    "in_stock": 50,
+    "allocated_build": 30,
+    "allocated_sales": 10,
+    "available": 10,
+    "required_build": 45,
+    "required_sales": 15,
+    "deficit": -10
+  }
+]
+```