test: split monolithic test_scan into a scan/ subpackage

Break the 1897-line test_scan.py into topic modules under tests/scan/
(routes, approve, properties, run) sharing fixtures via a package
conftest.py and pure builders via helpers.py. Same 80 tests, no
behavior change; full suite 627 passed.

ha-relevant: no

backend/tests/scan/helpers.py

@@ -0,0 +1,52 @@
+"""Shared builders for scan test suite (pure helpers, no fixtures)."""
+import uuid
+
+from app.db.models import Design, Node, PendingDevice
+
+
+async def _add_design(db_session, name: str) -> str:
+    design = Design(id=str(uuid.uuid4()), name=name)
+    db_session.add(design)
+    await db_session.commit()
+    return design.id
+
+
+def _node(design_id: str, *, ip=None, ieee=None, mac=None) -> Node:
+    return Node(
+        id=str(uuid.uuid4()), label="n", type="server", status="online",
+        ip=ip, mac=mac, ieee_address=ieee, services=[], pos_x=0.0, pos_y=0.0,
+        design_id=design_id,
+    )
+
+
+async def _seed_zigbee_pending_pair(db_session):
+    """Create a coordinator Node + a pending device + a link between them."""
+    from app.db.models import Node, PendingDeviceLink
+
+    coord = Node(
+        label="Coordinator",
+        type="zigbee_coordinator",
+        status="unknown",
+        ieee_address="0xCOORD",
+    )
+    db_session.add(coord)
+
+    pending = PendingDevice(
+        ieee_address="0xR1",
+        friendly_name="router_1",
+        suggested_type="zigbee_router",
+        device_subtype="Router",
+        status="pending",
+        discovery_source="zigbee",
+    )
+    db_session.add(pending)
+
+    db_session.add(
+        PendingDeviceLink(
+            source_ieee="0xCOORD",
+            target_ieee="0xR1",
+            discovery_source="zigbee",
+        )
+    )
+    await db_session.commit()
+    return coord, pending