Avoid full store list materialization in core fallback paths

[Copilot]

IStoreService.GetAllStores() was being used in multiple hot fallback paths that only needed a single store, causing full store collection loading into memory. This change introduces targeted store lookups and rewires key call sites to remove unnecessary full-list fetches.

• Service API changes

  • Added GetFirstStore() to IStoreService for single-store fallback retrieval.
  • Added dedicated cache keys:
    • Grand.stores.first
    • Grand.stores.host-{host}

• StoreService query/caching optimization

  • Implemented GetFirstStore() as an ordered single-record query.
  • Reworked GetStoreByHost(host) to query by host directly (with per-host cache), instead of loading all stores then filtering in memory.
  • Updated delete safety check to use existence semantics (Skip(1).Any()) rather than loading all stores.

• High-impact call-site rewiring

  • Replaced GetAllStores().FirstOrDefault() fallbacks with GetFirstStore() in:
    • store context initialization
    • work context initialization
    • campaign message store resolution
    • message provider fallback store resolution

• Example (pattern change)

  // before
  var store = await _storeService.GetStoreById(storeId) ??
              (await _storeService.GetAllStores()).FirstOrDefault();
  
  // after
  var store = await _storeService.GetStoreById(storeId) ??
              await _storeService.GetFirstStore();