feat(unit-swap): show benched candidates and clean stale modem pairings

`available-units` and `available-modems` now accept `include_benched=true`
to also return units/modems with `deployed=False`.  Default is False so
the existing location-detail swap modal is unchanged.  Each row carries
a `deployed` boolean for badge rendering.  The Unit Swap wizard fetches
with the flag enabled — exactly the candidates a field tech pulls off
the shelf.

The /swap endpoint now flips the incoming unit (and modem) back to
`deployed=True` when they came in benched, keeping the legacy roster
flag consistent with the active-assignment signal.

Adds the symmetric half of the orphan-pairing fix: when a newly-paired
modem still claims a different seismograph (whose
`deployed_with_modem_id` was never cleared in a past swap), break that
stale back-reference before re-pairing.

`locations-with-assignments` includes `modem.deployed` so the wizard
can badge the current modem in the location card, the "Keep current
modem" choice, the picker rows, and the review screen.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/routers/project_locations.py

@@ -318,6 +318,7 @@ async def get_locations_with_assignments(
                             "hardware_model": modem.hardware_model,
                             "ip_address":     modem.ip_address,
                             "phone_number":   modem.phone_number,
+                            "deployed":       bool(modem.deployed),
                         }
 
         results.append({
@@ -1306,12 +1307,28 @@ async def swap_unit_on_location(
         modem = db.query(RosterUnit).filter_by(id=modem_id, device_type="modem").first()
         if not modem:
             raise HTTPException(status_code=404, detail=f"Modem '{modem_id}' not found")
+        # Symmetric cleanup: if this modem still claims a previous partner
+        # (a different seismograph whose deployed_with_modem_id never got
+        # cleared in a past swap), break that stale link before re-pairing.
+        if modem.deployed_with_unit_id and modem.deployed_with_unit_id != unit_id:
+            prev_partner = db.query(RosterUnit).filter_by(id=modem.deployed_with_unit_id).first()
+            if prev_partner and prev_partner.deployed_with_modem_id == modem_id:
+                prev_partner.deployed_with_modem_id = None
         unit.deployed_with_modem_id = modem_id
         modem.deployed_with_unit_id = unit_id
+        # If the modem was on the bench, swapping it into the field puts it
+        # back in rotation.
+        if not modem.deployed:
+            modem.deployed = True
     else:
         # Clear modem pairing if not provided
         unit.deployed_with_modem_id = None
 
+    # If the incoming unit was benched, putting it in the field flips it
+    # back to deployed (so polling / dashboards see it as in rotation).
+    if not unit.deployed:
+        unit.deployed = True
+
     db.commit()
 
     return JSONResponse({
@@ -1329,23 +1346,32 @@ async def swap_unit_on_location(
 async def get_available_modems(
     project_id: str,
     db: Session = Depends(get_db),
+    include_benched: bool = Query(False),
 ):
     """
-    Get all deployed, non-retired modems for the modem assignment dropdown.
+    Get all non-retired modems for the modem assignment dropdown.
+
+    By default only deployed (in-rotation) modems are returned, preserving
+    the existing behavior for callers like the location-detail swap modal.
+    Pass ``include_benched=true`` to also include benched modems
+    (``RosterUnit.deployed == False``) — useful when picking a modem to
+    pull off the bench for a field swap.  Each row's ``deployed`` flag is
+    returned so the UI can badge benched candidates.
     """
-    modems = db.query(RosterUnit).filter(
-        and_(
-            RosterUnit.device_type == "modem",
-            RosterUnit.deployed == True,
-            RosterUnit.retired == False,
-        )
-    ).order_by(RosterUnit.id).all()
+    filters = [
+        RosterUnit.device_type == "modem",
+        RosterUnit.retired == False,
+    ]
+    if not include_benched:
+        filters.append(RosterUnit.deployed == True)
+    modems = db.query(RosterUnit).filter(and_(*filters)).order_by(RosterUnit.id).all()
 
     return [
         {
             "id": m.id,
             "hardware_model": m.hardware_model,
             "ip_address": m.ip_address,
+            "deployed": bool(m.deployed),
         }
         for m in modems
     ]
@@ -1356,22 +1382,31 @@ async def get_available_units(
     project_id: str,
     location_type: str = Query(...),
     db: Session = Depends(get_db),
+    include_benched: bool = Query(False),
 ):
     """
     Get list of available units for assignment to a location.
     Filters by device type matching the location type.
+
+    By default only deployed (in-rotation) units are returned, preserving
+    the existing location-detail swap-modal behavior.  Pass
+    ``include_benched=true`` to also include benched units
+    (``RosterUnit.deployed == False``) — exactly the candidates you'd
+    pull off the bench for a field swap.  Each row carries a ``deployed``
+    flag so the UI can badge benched picks.
     """
     # Determine required device type
     required_device_type = "slm" if location_type == "sound" else "seismograph"
 
-    # Get all units of the required type that are deployed and not retired
-    all_units = db.query(RosterUnit).filter(
-        and_(
-            RosterUnit.device_type == required_device_type,
-            RosterUnit.deployed == True,
-            RosterUnit.retired == False,
-        )
-    ).all()
+    # Get all units of the required type that aren't retired (and optionally
+    # exclude benched units).
+    filters = [
+        RosterUnit.device_type == required_device_type,
+        RosterUnit.retired == False,
+    ]
+    if not include_benched:
+        filters.append(RosterUnit.deployed == True)
+    all_units = db.query(RosterUnit).filter(and_(*filters)).all()
 
     # Filter out units that already have active assignments (active = assigned_until IS NULL)
     assigned_unit_ids = db.query(UnitAssignment.unit_id).filter(
@@ -1385,6 +1420,7 @@ async def get_available_units(
             "device_type": unit.device_type,
             "location": unit.address or unit.location,
             "model": unit.slm_model if unit.device_type == "slm" else unit.unit_type,
+            "deployed": bool(unit.deployed),
         }
         for unit in all_units
         if unit.id not in assigned_unit_ids