feat(core): add ContentServiceBase abstract class for Phase 1

Establishes shared infrastructure for content services:
- Common dependencies (DocumentRepository, AuditService, UserIdKeyResolver)
- Audit helper methods (sync and async)
- Inherits from RepositoryService for scope/query support
- Adds ContentServiceConstants for shared constants (batch page size)

Updated tracking test to look for correct assembly location.
Tracking test now fails (expected) - signals class exists for future work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/Umbraco.Core/Services/ContentServiceBase.cs

@@ -0,0 +1,69 @@
+using Microsoft.Extensions.Logging;
+using Umbraco.Cms.Core.Events;
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Persistence.Querying;
+using Umbraco.Cms.Core.Persistence.Repositories;
+using Umbraco.Cms.Core.Scoping;
+
+namespace Umbraco.Cms.Core.Services;
+
+/// <summary>
+/// Abstract base class for content-related services providing shared infrastructure.
+/// </summary>
+public abstract class ContentServiceBase : RepositoryService
+{
+    protected readonly IDocumentRepository DocumentRepository;
+    protected readonly IAuditService AuditService;
+    protected readonly IUserIdKeyResolver UserIdKeyResolver;
+
+    protected ContentServiceBase(
+        ICoreScopeProvider provider,
+        ILoggerFactory loggerFactory,
+        IEventMessagesFactory eventMessagesFactory,
+        IDocumentRepository documentRepository,
+        IAuditService auditService,
+        IUserIdKeyResolver userIdKeyResolver)
+        : base(provider, loggerFactory, eventMessagesFactory)
+    {
+        DocumentRepository = documentRepository ?? throw new ArgumentNullException(nameof(documentRepository));
+        AuditService = auditService ?? throw new ArgumentNullException(nameof(auditService));
+        UserIdKeyResolver = userIdKeyResolver ?? throw new ArgumentNullException(nameof(userIdKeyResolver));
+    }
+
+    /// <summary>
+    /// Records an audit entry for a content operation (synchronous).
+    /// </summary>
+    /// <remarks>
+    /// Uses ConfigureAwait(false) to avoid capturing synchronization context and prevent deadlocks.
+    /// TODO: Replace with sync overloads when IAuditService.Add and IUserIdKeyResolver.Get are available.
+    /// </remarks>
+    protected void Audit(AuditType type, int userId, int objectId, string? message = null, string? parameters = null)
+    {
+        // Use ConfigureAwait(false) to avoid context capture and potential deadlocks
+        Guid userKey = UserIdKeyResolver.GetAsync(userId).ConfigureAwait(false).GetAwaiter().GetResult();
+
+        AuditService.AddAsync(
+            type,
+            userKey,
+            objectId,
+            UmbracoObjectTypes.Document.GetName(),
+            message,
+            parameters).ConfigureAwait(false).GetAwaiter().GetResult();
+    }
+
+    /// <summary>
+    /// Records an audit entry for a content operation asynchronously.
+    /// </summary>
+    protected async Task AuditAsync(AuditType type, int userId, int objectId, string? message = null, string? parameters = null)
+    {
+        Guid userKey = await UserIdKeyResolver.GetAsync(userId).ConfigureAwait(false);
+
+        await AuditService.AddAsync(
+            type,
+            userKey,
+            objectId,
+            UmbracoObjectTypes.Document.GetName(),
+            message,
+            parameters).ConfigureAwait(false);
+    }
+}

src/Umbraco.Core/Services/ContentServiceConstants.cs

@@ -0,0 +1,12 @@
+namespace Umbraco.Cms.Core.Services;
+
+/// <summary>
+/// Constants used by content-related services.
+/// </summary>
+public static class ContentServiceConstants
+{
+    /// <summary>
+    /// Default page size for batch operations (e.g., cascade delete).
+    /// </summary>
+    public const int DefaultBatchPageSize = 500;
+}