feat(vision): immagini di riferimento, per comporre senza camera

VisionReferenceImage (migration 013, chained after 012_vision_results):
task_id, path, station_id, device_code, calibration_snapshot,
engine_version, note, expected_json, acquired_at. station_id/device_code/
calibration_snapshot/engine_version/expected_json stay nullable - nessun
endpoint di questo task li popola ancora, arrivano coi Piani 1b/2/4.

save_reference_image salva sotto settings.upload_path (non upload_dir,
come production_export_service). preview esegue il grafo e non salva
niente - comporre non e' misurare, lo asserisce il test.

POST /api/vision/reference-images e /api/vision/preview su require_maker
(non require_measurement_tec di execute): comporre e provare sono lavoro
del Maker, non misure.

Suite intera su Python 3.11.15 locale (uv run pytest -q, SQLite
in-memory per il backend): 1 failed, 373 passed, 4 skipped in 82.35s.
Il fallimento e' quello preesistente e non correlato di
test_no_first_party_script_calls_out (fabric-debug.js locale,
git-ignored) - 371 passed prima di questo task, +2 qui.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014BBnuACZSCJqXrMYC3LUMU

src/backend/models/orm/vision.py

@@ -38,3 +38,32 @@ class VisionResult(Base):
 
     def __repr__(self) -> str:
         return f"<VisionResult task={self.task_id} engine={self.engine_version}>"
+
+
+class VisionReferenceImage(Base):
+    """An image kept so a graph can be composed, tried and re-tried without a
+    camera - which is exactly what VisionSuite's source abstraction is for."""
+
+    __tablename__ = "vision_reference_images"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    task_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("recipe_tasks.id"), nullable=False, index=True
+    )
+    path: Mapped[str] = mapped_column(String(500), nullable=False)
+    station_id: Mapped[Optional[int]] = mapped_column(
+        Integer, ForeignKey("stations.id"), nullable=True
+    )
+    device_code: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
+    calibration_snapshot: Mapped[Optional[dict]] = mapped_column(JSON, nullable=True)
+    engine_version: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    note: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
+    expected_json: Mapped[Optional[dict]] = mapped_column(JSON, nullable=True)
+    acquired_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, server_default=func.now()
+    )
+
+    __table_args__ = ({"mysql_engine": "InnoDB", "mysql_charset": "utf8mb4"},)
+
+    def __repr__(self) -> str:
+        return f"<VisionReferenceImage task={self.task_id} path={self.path}>"