feat: support image inputs in assistant tests

backend/db/models.py

@@ -475,6 +475,23 @@ class TestSuite(Base):
     )
 
 
+class TestAsset(Base):
+    """Immutable image fixture referenced by one or more test definitions."""
+
+    __tablename__ = "test_assets"
+
+    id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    original_name: Mapped[str] = mapped_column(String(255), default="image.jpg")
+    storage_key: Mapped[str] = mapped_column(String(1024), unique=True)
+    mime_type: Mapped[str] = mapped_column(String(128), default="image/jpeg")
+    width: Mapped[int] = mapped_column(Integer)
+    height: Mapped[int] = mapped_column(Integer)
+    size_bytes: Mapped[int] = mapped_column(Integer)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+
 class TestCase(Base):
     """One editable test definition; nested turns remain one JSON document."""