Separate unit placements from shift duty assignments.

Roster no longer writes temporary staff assignments; unit assignment UI drops shift fields and labels clarify the two workflows.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/Models/StaffAssignment.php

@@ -7,6 +7,7 @@ use Carbon\CarbonInterface;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\SoftDeletes;
 
 class StaffAssignment extends Model
@@ -58,6 +59,20 @@ class StaffAssignment extends Model
         return $this->belongsTo(CareUnit::class, 'care_unit_id');
     }
 
+    /**
+     * Legacy link from when duty roster auto-created unit placements.
+     * Shift assignments live on RosterEntry; new roster rows do not set this.
+     */
+    public function rosterEntries(): HasMany
+    {
+        return $this->hasMany(RosterEntry::class, 'staff_assignment_id');
+    }
+
+    public function scopeUnitPlacements(Builder $query): Builder
+    {
+        return $query->whereDoesntHave('rosterEntries');
+    }
+
     public function scopeEffectiveOn(Builder $query, CarbonInterface|string|null $date = null): Builder
     {
         $day = $date