Performance: push ORDER BY/TAKE into SQL for hot-path reads

- IInAppNotificationRepository: typed repo with GetPagedAsync, GetRecentAsync, GetUnreadAsync
  — bell dropdown no longer loads all notifications then slices in C#
- Add compound indexes on InAppNotifications(CompanyId, IsDeleted, CreatedAt) and
  (CompanyId, IsDeleted, IsRead); ContactSubmissions(CompanyId, IsDeleted, CreatedAt)
- PlainRepository.GetAllAsync/FindAsync: add AsNoTracking (Announcements, Tips, ReleaseNotes)
- AiUsageReportController: replace GetAllAsync + C# Where with FindAsync (SQL-level filter)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Infrastructure/Repositories/UnitOfWork.cs

@@ -109,7 +109,7 @@ public class UnitOfWork : IUnitOfWork
     private IPlainRepository<Announcement>? _announcements;
     private IPlainRepository<BannedIp>? _bannedIps;
     private IPlainRepository<DashboardTip>? _dashboardTips;
-    private IRepository<InAppNotification>? _inAppNotifications;
+    private IInAppNotificationRepository? _inAppNotifications;
     private IPlainRepository<ReleaseNote>? _releaseNotes;
 
     // Bug Reports
@@ -439,8 +439,8 @@ public class UnitOfWork : IUnitOfWork
         _dashboardTips ??= new PlainRepository<DashboardTip>(_context);
 
     /// <summary>Repository for <see cref="InAppNotification"/> bell-notification records; tenant-filtered with soft delete.</summary>
-    public IRepository<InAppNotification> InAppNotifications =>
-        _inAppNotifications ??= new Repository<InAppNotification>(_context);
+    public IInAppNotificationRepository InAppNotifications =>
+        _inAppNotifications ??= new InAppNotificationRepository(_context);
 
     /// <summary>Repository for <see cref="ReleaseNote"/> platform changelog entries; no tenant filter, no soft delete.</summary>
     public IPlainRepository<ReleaseNote> ReleaseNotes =>