Add region manager assignment endpoints

Distribution/Features/Region/RegionService.cs

@@ -91,6 +91,67 @@ public class RegionService(IRegionRepository repo, IUnitOfWork uow, ICurrentUser
         return new RegionResponse(region.Id, region.Code, region.Name, region.IsActive, region.CreatedAt);
     }
 
+    private const string RegionalManagerRole = "RegionalManager";
+
+    public async Task<RegionManagerResponse?> GetManagerAsync(Guid regionId)
+    {
+        var region = await repo.GetByIdAsync(regionId);
+        if (region is null || !region.IsActive) return null;
+        var scope = (await repo.GetManagerScopesAsync(regionId)).FirstOrDefault();
+        return scope is null ? null : new RegionManagerResponse(regionId, scope.FK_UserId, scope.User.Username, scope.User.FullName);
+    }
+
+    // A region has exactly one manager: assigning replaces any existing RegionalManager scoped to it.
+    // Returns null when the region does not exist; throws InvalidOperationException for an invalid user.
+    public async Task<RegionManagerResponse?> SetManagerAsync(Guid regionId, SetRegionManagerRequest request, CancellationToken ct = default)
+    {
+        var region = await repo.GetByIdAsync(regionId);
+        if (region is null || !region.IsActive) return null;
+
+        var user = await repo.GetUserWithRoleAsync(request.UserId);
+        if (user is null || !user.IsActive)
+            throw new InvalidOperationException("User not found.");
+        if (user.Role?.Name != RegionalManagerRole)
+            throw new InvalidOperationException($"User must have the {RegionalManagerRole} role to manage a region.");
+
+        await uow.BeginAsync(ct);
+        try
+        {
+            var existing = await repo.GetManagerScopesAsync(regionId);
+            repo.RemoveRegionScopes(existing.Where(s => s.FK_UserId != user.Id));
+            if (await repo.GetRegionScopeAsync(user.Id, regionId) is null)
+                await repo.AddRegionScopeAsync(new Models.UserRegionScope { FK_UserId = user.Id, FK_RegionId = regionId });
+            await uow.CommitAsync(ct);
+        }
+        catch
+        {
+            await uow.RollbackAsync(ct);
+            throw;
+        }
+        return new RegionManagerResponse(regionId, user.Id, user.Username, user.FullName);
+    }
+
+    public async Task<bool> RemoveManagerAsync(Guid regionId, CancellationToken ct = default)
+    {
+        var region = await repo.GetByIdAsync(regionId);
+        if (region is null || !region.IsActive) return false;
+        var existing = await repo.GetManagerScopesAsync(regionId);
+        if (existing.Count == 0) return false;
+
+        await uow.BeginAsync(ct);
+        try
+        {
+            repo.RemoveRegionScopes(existing);
+            await uow.CommitAsync(ct);
+        }
+        catch
+        {
+            await uow.RollbackAsync(ct);
+            throw;
+        }
+        return true;
+    }
+
     public async Task<bool> DeleteAsync(Guid id, CancellationToken ct = default)
     {
         var region = await repo.GetByIdAsync(id);