feat: test version of unit swap tool.

backend/main.py

@@ -291,6 +291,14 @@ async def pending_deployments_page(request: Request):
     return templates.TemplateResponse("admin/pending_deployments.html", {"request": request})
 
 
+@app.get("/tools/unit-swap", response_class=HTMLResponse)
+async def unit_swap_page(request: Request):
+    """Mobile-first wizard for swapping a vibration unit (and optionally its
+    modem) at a monitoring location.  Pick project → location → incoming
+    unit → modem decision → confirm → optional photo of the new install."""
+    return templates.TemplateResponse("admin/unit_swap.html", {"request": request})
+
+
 @app.get("/modems", response_class=HTMLResponse)
 async def modems_page(request: Request):
     """Field modems management dashboard"""

backend/routers/project_locations.py

@@ -262,6 +262,83 @@ async def get_project_locations_json(
     ]
 
 
+@router.get("/locations-with-assignments")
+async def get_locations_with_assignments(
+    project_id: str,
+    db: Session = Depends(get_db),
+    location_type: Optional[str] = Query(None),
+):
+    """
+    Locations + their currently-active assignment + current unit + paired modem
+    in one call.  Used by the Unit Swap tool's location picker so a field tech
+    can see what's deployed where without N+1 round-trips.
+
+    Empty locations come back with assignment/unit/modem all null.
+    Removed locations are always excluded — you don't swap onto a dead slot.
+    """
+    project = db.query(Project).filter_by(id=project_id).first()
+    if not project:
+        raise HTTPException(status_code=404, detail="Project not found")
+
+    query = db.query(MonitoringLocation).filter_by(project_id=project_id).filter(
+        MonitoringLocation.removed_at == None  # noqa: E711
+    )
+    if location_type:
+        query = query.filter_by(location_type=location_type)
+    locations = query.order_by(MonitoringLocation.sort_order, MonitoringLocation.name).all()
+
+    results = []
+    for loc in locations:
+        assignment = db.query(UnitAssignment).filter(
+            and_(
+                UnitAssignment.location_id == loc.id,
+                UnitAssignment.assigned_until == None,  # noqa: E711
+            )
+        ).first()
+
+        unit_payload = None
+        modem_payload = None
+        if assignment:
+            unit = db.query(RosterUnit).filter_by(id=assignment.unit_id).first()
+            if unit:
+                unit_payload = {
+                    "id":           unit.id,
+                    "device_type":  unit.device_type,
+                    "unit_type":    unit.unit_type,
+                    "slm_model":    unit.slm_model,
+                    "deployed_with_modem_id": unit.deployed_with_modem_id,
+                }
+                if unit.deployed_with_modem_id:
+                    modem = db.query(RosterUnit).filter_by(
+                        id=unit.deployed_with_modem_id, device_type="modem"
+                    ).first()
+                    if modem:
+                        modem_payload = {
+                            "id":             modem.id,
+                            "hardware_model": modem.hardware_model,
+                            "ip_address":     modem.ip_address,
+                            "phone_number":   modem.phone_number,
+                        }
+
+        results.append({
+            "id":            loc.id,
+            "name":          loc.name,
+            "location_type": loc.location_type,
+            "description":   loc.description,
+            "address":       loc.address,
+            "coordinates":   loc.coordinates,
+            "assignment": {
+                "id":          assignment.id,
+                "assigned_at": assignment.assigned_at.isoformat() if assignment.assigned_at else None,
+                "notes":       assignment.notes,
+            } if assignment else None,
+            "unit":  unit_payload,
+            "modem": modem_payload,
+        })
+
+    return results
+
+
 @router.post("/locations/create")
 async def create_location(
     project_id: str,
@@ -1192,6 +1269,17 @@ async def swap_unit_on_location(
                 new_value=f"swapped out → {unit_id}",
                 notes=notes,
             )
+            # Clear the outgoing unit's modem pairing so the bidirectional
+            # deployed_with_modem_id / deployed_with_unit_id back-reference
+            # doesn't orphan onto the unit that just left the field.
+            old_unit = db.query(RosterUnit).filter_by(id=current.unit_id).first()
+            if old_unit and old_unit.deployed_with_modem_id:
+                old_modem = db.query(RosterUnit).filter_by(
+                    id=old_unit.deployed_with_modem_id, device_type="modem"
+                ).first()
+                if old_modem and old_modem.deployed_with_unit_id == current.unit_id:
+                    old_modem.deployed_with_unit_id = None
+                old_unit.deployed_with_modem_id = None
 
     # Create new assignment
     new_assignment = UnitAssignment(