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>

.gitignore

@@ -0,0 +1,15 @@
+# Runtime data (uploads, generated artifacts, SQLite DB)
+_data/
+# Phase scratch (build/smoke/api-test logs + outputs)
+_phase0_out/
+_phase*.log
+# Python
+__pycache__/
+*.pyc
+.venv/
+venv/
+# Env / secrets
+.env
+# OS
+.DS_Store
+**/.DS_Store