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/Services/CapacityService.php

@@ -273,6 +273,21 @@ class CapacityService
             ->mapWithKeys(fn (TeamMemberAvailability $entry) => [$entry->date->toDateString() => (float) $entry->availability]);
     }
 
+    public function upsertTeamMemberAvailability(string $teamMemberId, string $date, float $availability): TeamMemberAvailability
+    {
+        $entry = TeamMemberAvailability::updateOrCreate(
+            ['team_member_id' => $teamMemberId, 'date' => $date],
+            ['availability' => $availability]
+        );
+
+        $month = Carbon::createFromFormat('Y-m-d', $date)->format('Y-m');
+
+        $this->forgetCapacityCacheForTeamMember($teamMemberId, [$month]);
+        $this->forgetCapacityCacheForMonth($month);
+
+        return $entry;
+    }
+
     /**
      * Create a CarbonPeriod for the given month.
      */