feat(vision): il server esegue tramite worker e salva misure vere

Aggiunge POST /api/vision/execute: route l'immagine al worker di visione,
mappa le uscite del grafo sulle quote e le salva con save_measurement -
stesso verdetto, stesso gate del fuori tolleranza di ogni altra misura.
Nuova tabella vision_results (una riga per acquisizione, non per quota) e
input_method 'camera' su measurements.

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

src/backend/models/orm/measurement.py

@@ -38,11 +38,17 @@ class Measurement(Base):
 
     # Input method
     input_method: Mapped[str] = mapped_column(
-        Enum("usb_caliper", "manual", name="input_method_enum"),
+        Enum("usb_caliper", "manual", "camera", name="input_method_enum"),
         nullable=False,
         default="manual",
     )
 
+    # The vision execution that produced this value, when there was one. Null for
+    # everything taken by caliper or typed by hand.
+    vision_result_id: Mapped[Optional[int]] = mapped_column(
+        Integer, ForeignKey("vision_results.id"), nullable=True
+    )
+
     # Time spent by the operator entering this value (client-reported)
     input_duration_ms: Mapped[Optional[int]] = mapped_column(
         Integer, nullable=True