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/models.py

@@ -0,0 +1,64 @@
+"""SQLModel tables: Model, Job, BomRow, QueryLog."""
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlmodel import Field, SQLModel
+
+
+def utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+class Model(SQLModel, table=True):
+    """One uploaded STEP file and its extracted metadata."""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str
+    original_filename: str
+    stem: str
+    backend: Optional[str] = None          # "build123d" | "freecad"
+    face_count: Optional[int] = None
+    part_count: Optional[int] = None
+    bbox_x_mm: Optional[float] = None
+    bbox_y_mm: Optional[float] = None
+    bbox_z_mm: Optional[float] = None
+    has_chinese: bool = False
+    created_at: datetime = Field(default_factory=utcnow)
+
+
+class Job(SQLModel, table=True):
+    """A processing run for a model. Status: pending|running|done|error."""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    model_id: int = Field(foreign_key="model.id", index=True)
+    status: str = Field(default="pending", index=True)
+    stage: Optional[str] = None            # current pipeline stage
+    error: Optional[str] = None
+    artifacts: Optional[str] = None        # JSON list of relative filenames
+    options: Optional[str] = None          # JSON of the request options
+    created_at: datetime = Field(default_factory=utcnow)
+    started_at: Optional[datetime] = None
+    finished_at: Optional[datetime] = None
+
+
+class BomRow(SQLModel, table=True):
+    """One BOM line for a model."""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    model_id: int = Field(foreign_key="model.id", index=True)
+    part_number: Optional[str] = None
+    part_name_original: Optional[str] = None
+    part_name_english: Optional[str] = None
+    quantity: Optional[int] = None
+    level: Optional[int] = None
+    parent: Optional[str] = None
+    bbox_x_mm: Optional[float] = None
+    bbox_y_mm: Optional[float] = None
+    bbox_z_mm: Optional[float] = None
+    notes: Optional[str] = None
+
+
+class QueryLog(SQLModel, table=True):
+    """A natural-language geometry query and its result."""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    model_id: int = Field(foreign_key="model.id", index=True)
+    query: str
+    result: Optional[str] = None
+    created_at: datetime = Field(default_factory=utcnow)