feat(unit-detail): editable deployment timeline

Each assignment row in the timeline now gets an inline edit (pencil)
that opens a modal with `assigned_at`, `assigned_until`, and notes.
Save calls the existing `PATCH /api/projects/{pid}/assignments/{aid}`;
delete (for misclicks) calls the existing `DELETE`.  Open-ended
checkbox clears `assigned_until` and the endpoint flips status back
to "active".

Adds an "+ Add deployment record" button at the top of the timeline
for backfilling historical windows when orphan events sit outside any
assignment.  Modal: project → location → assigned_at → assigned_until
(optional open-ended) → notes.

Backend: the `/locations/{loc}/assign` endpoint now accepts an
`assigned_at` form field and a closed-window assignment.  The previous
blanket "location already has an active assignment" check is replaced
with same-location overlap detection — closed historical windows that
don't overlap an existing assignment are accepted (which is exactly
the backfill case).

After any save/delete the timeline reloads and the SFM-events list
re-fetches so previously-orphaned events flip to "attributed" when
their timestamp now falls inside an assignment window.

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

backend/routers/project_locations.py

@@ -723,6 +723,19 @@ async def assign_unit_to_location(
 ):
     """
     Assign a unit to a monitoring location.
+
+    Accepts form fields:
+      - unit_id        — required
+      - assigned_at    — optional ISO datetime; defaults to now.  Set this
+                         when backfilling a historical deployment whose
+                         events landed in the orphan bucket.
+      - assigned_until — optional ISO datetime; absent = open-ended /
+                         active.
+      - notes          — optional free text
+
+    Refuses only when the *new window would overlap* an existing active
+    open-ended assignment at the same location.  Closed historical windows
+    that don't overlap are allowed (and required for orphan-event backfill).
     """
     location = db.query(MonitoringLocation).filter_by(
         id=location_id,
@@ -748,23 +761,55 @@ async def assign_unit_to_location(
             detail=f"Unit type '{unit.device_type}' does not match location type '{location.location_type}'",
         )
 
-    # Check if location already has an active assignment (active = assigned_until IS NULL)
-    existing_assignment = db.query(UnitAssignment).filter(
-        and_(
-            UnitAssignment.location_id == location_id,
-            UnitAssignment.assigned_until == None,
-        )
-    ).first()
-
-    if existing_assignment:
-        raise HTTPException(
-            status_code=400,
-            detail=f"Location already has an active unit assignment ({existing_assignment.unit_id}). Use swap to replace it.",
-        )
-
-    # Create new assignment
+    # Parse dates.
+    assigned_at_str    = form_data.get("assigned_at")
     assigned_until_str = form_data.get("assigned_until")
-    assigned_until = datetime.fromisoformat(assigned_until_str) if assigned_until_str else None
+    try:
+        assigned_at = (
+            datetime.fromisoformat(assigned_at_str)
+            if assigned_at_str else datetime.utcnow()
+        )
+    except (TypeError, ValueError):
+        raise HTTPException(
+            status_code=400, detail=f"Invalid assigned_at: {assigned_at_str!r}"
+        )
+    try:
+        assigned_until = (
+            datetime.fromisoformat(assigned_until_str)
+            if assigned_until_str else None
+        )
+    except (TypeError, ValueError):
+        raise HTTPException(
+            status_code=400, detail=f"Invalid assigned_until: {assigned_until_str!r}"
+        )
+    if assigned_until is not None and assigned_until <= assigned_at:
+        raise HTTPException(
+            status_code=400, detail="assigned_until must be after assigned_at.",
+        )
+
+    # Reject only if the new window overlaps an existing assignment at the
+    # SAME location.  Closed historical windows that sit before the current
+    # active assignment are fine — that's the backfill case.
+    new_end_for_overlap = assigned_until or datetime.utcnow()
+    existing = db.query(UnitAssignment).filter(
+        UnitAssignment.location_id == location_id
+    ).all()
+    for other in existing:
+        other_start = other.assigned_at
+        other_end   = other.assigned_until or datetime.utcnow()
+        if assigned_at < other_end and new_end_for_overlap > other_start:
+            other_window = (
+                f"{other.assigned_at:%Y-%m-%d}"
+                + (f" → {other.assigned_until:%Y-%m-%d}" if other.assigned_until else " → present")
+            )
+            raise HTTPException(
+                status_code=400,
+                detail=(
+                    f"New window overlaps an existing assignment at this "
+                    f"location ({other.unit_id} {other_window}). Use swap or "
+                    f"edit that record instead."
+                ),
+            )
 
     assignment = UnitAssignment(
         id=str(uuid.uuid4()),
@@ -772,8 +817,9 @@ async def assign_unit_to_location(
         location_id=location_id,
         project_id=project_id,
         device_type=unit.device_type,
+        assigned_at=assigned_at,
         assigned_until=assigned_until,
-        status="active",
+        status="active" if assigned_until is None else "completed",
         notes=form_data.get("notes"),
     )