fix(capacity): Fix three defects - caching, save, filters

1. Slow Team Member Dropdown - Fixed
   - Added cached team members store with 5-minute TTL
   - Dropdown now loads instantly on subsequent visits

2. Error Preventing Capacity Save - Fixed
   - Added saveAvailability API endpoint
   - Added backend service method to persist availability overrides
   - Added proper error handling and success feedback
   - Cache invalidation on save

3. Filters Not Working - Fixed
   - Fixed PTOManager to use shared selectedMemberId
   - Filters now react to team member selection

Test Results:
- Backend: 76 passed ✅
- Frontend Unit: 10 passed ✅
- E2E: 130 passed, 24 skipped ✅

Refs: openspec/changes/headroom-foundation

backend/app/Http/Resources/TeamMemberAvailabilityResource.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Http\Resources;
+
+use App\Models\TeamMemberAvailability;
+
+class TeamMemberAvailabilityResource extends BaseResource
+{
+    public function toArray($request): array
+    {
+        /** @var TeamMemberAvailability $availability */
+        $availability = $this->resource;
+
+        return [
+            'team_member_id' => $availability->team_member_id,
+            'date' => $availability->date?->toDateString(),
+            'availability' => (float) $availability->availability,
+        ];
+    }
+}