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>

backend/app/db.py

@@ -0,0 +1,28 @@
+"""SQLite engine + session helpers.
+
+SQLModel (SQLAlchemy + Pydantic) is used rather than Jason's usual Prisma — Prisma
+is JS-first and its Python client is unofficial, whereas SQLModel is the FastAPI-
+native ORM. SQLite stays the DB, single-file under the data volume.
+"""
+from sqlmodel import SQLModel, Session, create_engine
+
+from .config import DATA_DIR, DB_PATH, MODELS_DIR
+
+# check_same_thread=False: the worker thread shares the engine with request handlers.
+engine = create_engine(
+    f"sqlite:///{DB_PATH}",
+    connect_args={"check_same_thread": False},
+)
+
+
+def init_db() -> None:
+    DATA_DIR.mkdir(parents=True, exist_ok=True)
+    MODELS_DIR.mkdir(parents=True, exist_ok=True)
+    # Import models so they register on SQLModel.metadata before create_all.
+    from . import models  # noqa: F401
+    SQLModel.metadata.create_all(engine)
+
+
+def get_session():
+    with Session(engine) as session:
+        yield session