feat(sfm): editable UnitAssignment date windows (backdate deployments)

Operators couldn't change a unit's assigned_at / assigned_until after
creating the assignment, so a unit physically deployed in December 2025
but only recorded in terra-view today would show "deployed today" and
all its real events would be invisible on the project's location page.

Backend:
- PATCH /api/projects/{project_id}/assignments/{assignment_id}
  Accepts JSON body with optional assigned_at, assigned_until, notes.
  - assigned_at is required (cannot be cleared)
  - assigned_until can be null to mark active / indefinite
  - assigned_until must be after assigned_at
  - rejects overlaps with other assignments of the same unit at the
    same location (different units overlapping is fine — that's a
    legitimate swap window)
  - assignment.status flips to "active" when assigned_until is cleared,
    "completed" when set
  - 404 if the assignment doesn't belong to {project_id} (security)

Frontend (vibration_location_detail.html):
- Pencil icon next to each row in the "Seismographs deployed at this
  location" card. Click to open a modal with datetime-local inputs for
  From + Until (blank = active) and a Notes textarea. Save reloads the
  Events tab so KPI tiles and the event table reflect the new window.
- Helper line under the assignment list explains the workflow:
  "Click the pencil to backdate a deployment so historical events get
  attributed to this location."

Verified end-to-end against real data: backdating BE11529's assignment
on a vibration location from 2026-04-14 to 2025-12-01 surfaced 10
additional events (24 -> 34) that were previously invisible.

Validation suite (all returning correct HTTP codes):
  - assigned_until < assigned_at -> 400
  - cross-project assignment_id -> 404
  - assigned_at cleared -> 400
  - notes-only update -> 200

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

backend/routers/project_locations.py

@@ -453,6 +453,134 @@ async def unassign_unit(
     return {"success": True, "message": "Unit unassigned successfully"}
 
 
+@router.patch("/assignments/{assignment_id}")
+async def update_assignment(
+    project_id: str,
+    assignment_id: str,
+    request: Request,
+    db: Session = Depends(get_db),
+):
+    """
+    Update an assignment's date window and/or notes.
+
+    Common use case: backdate a deployment so events emitted before the
+    operator created the assignment in terra-view (e.g. a unit that was
+    physically deployed in December but only recorded in the system today)
+    get correctly attributed to the location.
+
+    Accepts JSON body with optional fields:
+      - assigned_at:    ISO datetime (or empty string to leave unchanged)
+      - assigned_until: ISO datetime, or null/"" to mark indefinite (active)
+      - notes:          string
+
+    Sets `status` to "active" when assigned_until is cleared, "completed"
+    when it's set in the past.
+    """
+    assignment = db.query(UnitAssignment).filter_by(
+        id=assignment_id,
+        project_id=project_id,
+    ).first()
+
+    if not assignment:
+        raise HTTPException(status_code=404, detail="Assignment not found")
+
+    try:
+        payload = await request.json()
+    except Exception:
+        raise HTTPException(status_code=400, detail="Invalid JSON body")
+
+    # Parse new values (None = unchanged, explicit None/"" for assigned_until = clear)
+    new_assigned_at = assignment.assigned_at
+    new_assigned_until = assignment.assigned_until
+    new_notes = assignment.notes
+
+    if "assigned_at" in payload:
+        raw = payload["assigned_at"]
+        if raw is None or raw == "":
+            raise HTTPException(
+                status_code=400,
+                detail="assigned_at is required; cannot be cleared.",
+            )
+        try:
+            # Accept "YYYY-MM-DDTHH:MM" from datetime-local inputs or full ISO.
+            new_assigned_at = datetime.fromisoformat(raw)
+        except (TypeError, ValueError):
+            raise HTTPException(
+                status_code=400,
+                detail=f"Invalid assigned_at datetime: {raw!r}",
+            )
+
+    if "assigned_until" in payload:
+        raw = payload["assigned_until"]
+        if raw is None or raw == "":
+            new_assigned_until = None
+        else:
+            try:
+                new_assigned_until = datetime.fromisoformat(raw)
+            except (TypeError, ValueError):
+                raise HTTPException(
+                    status_code=400,
+                    detail=f"Invalid assigned_until datetime: {raw!r}",
+                )
+
+    if "notes" in payload:
+        raw = payload["notes"]
+        new_notes = (raw or "").strip() or None
+
+    # Validation: end must be after start if both set.
+    if new_assigned_until is not None and new_assigned_until <= new_assigned_at:
+        raise HTTPException(
+            status_code=400,
+            detail="assigned_until must be after assigned_at.",
+        )
+
+    # Sanity: reject creating an overlap with another assignment of the SAME
+    # unit at the SAME location.  Different units at the same location can
+    # legitimately overlap during a swap window (rare but valid).
+    new_end_for_overlap = new_assigned_until or datetime.utcnow()
+    overlapping = (
+        db.query(UnitAssignment)
+        .filter(UnitAssignment.location_id == assignment.location_id)
+        .filter(UnitAssignment.unit_id == assignment.unit_id)
+        .filter(UnitAssignment.id != assignment.id)
+        .all()
+    )
+    for other in overlapping:
+        other_start = other.assigned_at
+        other_end = other.assigned_until or datetime.utcnow()
+        if new_assigned_at < other_end and new_end_for_overlap > other_start:
+            raise HTTPException(
+                status_code=400,
+                detail=(
+                    f"This window overlaps with another assignment for the "
+                    f"same unit ({other.assigned_at:%Y-%m-%d} → "
+                    f"{other.assigned_until and other.assigned_until.strftime('%Y-%m-%d') or 'present'})."
+                ),
+            )
+
+    # Apply.
+    assignment.assigned_at = new_assigned_at
+    assignment.assigned_until = new_assigned_until
+    assignment.notes = new_notes
+    assignment.status = "completed" if new_assigned_until is not None else "active"
+
+    db.commit()
+    db.refresh(assignment)
+
+    return {
+        "success": True,
+        "assignment": {
+            "id":             assignment.id,
+            "unit_id":        assignment.unit_id,
+            "location_id":    assignment.location_id,
+            "assigned_at":    assignment.assigned_at.isoformat() if assignment.assigned_at else None,
+            "assigned_until": assignment.assigned_until.isoformat() if assignment.assigned_until else None,
+            "status":         assignment.status,
+            "notes":          assignment.notes,
+        },
+    }
+
+
 @router.post("/locations/{location_id}/swap")
 async def swap_unit_on_location(
     project_id: str,