Phase 1: FastAPI backend with async job model

- backend/app: FastAPI API wrapping the CAD skill modules
  - upload -> job -> poll -> model / BOM / artifacts -> geometry query
  - SQLite via SQLModel (Model, Job, BomRow, QueryLog)
  - ThreadPoolExecutor worker, serialized, with live stage updates
- docker-compose.yml: dev server (mounts source, --reload) on :8000
- api-test.sh: end-to-end live validation script
- requirements.txt: add fastapi, uvicorn, python-multipart, sqlmodel
- external_diagram.py: port active-area detection OCC.Core -> OCP
- .gitignore, PHASE1.md

Validated live: MR16 round-trip passes (28 BOM rows, 12 artifacts,
bounding-box query, xlsx download; active-area detection working).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skill.src/modules/external_diagram.py

@@ -873,10 +873,13 @@ def _detect_active_area(model: StepModel, selected_parts: list) -> dict | None:
     """
     try:
         if model.backend == "build123d":
-            from OCC.Core.BRepGProp import BRepGProp
-            from OCC.Core.GProp import GProp_GProps
-            from OCC.Core.GeomAdaptor import GeomAdaptor_Surface
-            from OCC.Core.GeomAbs import GeomAbs_Plane
+            # build123d ships the OpenCASCADE kernel as OCP (cadquery-ocp), not the
+            # pythonocc `OCC.Core` package. OCP mirrors the same module names and the
+            # same `_s` static-method suffix, so this is a 1:1 import rename.
+            from OCP.BRepGProp import BRepGProp
+            from OCP.GProp import GProp_GProps
+            from OCP.GeomAdaptor import GeomAdaptor_Surface
+            from OCP.GeomAbs import GeomAbs_Plane
 
             best_area = 0
             best_face_data = None
@@ -885,7 +888,7 @@ def _detect_active_area(model: StepModel, selected_parts: list) -> dict | None:
 
             for face in model.shape.faces():
                 try:
-                    from OCC.Core.BRep import BRep_Tool
+                    from OCP.BRep import BRep_Tool
                     surf = BRep_Tool.Surface_s(face.wrapped)
                     adaptor = GeomAdaptor_Surface(surf)
                     if adaptor.GetType() != GeomAbs_Plane: