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/routes/api.php

@@ -45,6 +45,7 @@ Route::middleware(JwtAuth::class)->group(function () {
     Route::get('/capacity', [CapacityController::class, 'individual']);
     Route::get('/capacity/team', [CapacityController::class, 'team']);
     Route::get('/capacity/revenue', [CapacityController::class, 'revenue']);
+    Route::post('/capacity/availability', [CapacityController::class, 'saveAvailability']);
 
     // Holidays
     Route::get('/holidays', [HolidayController::class, 'index']);