feat(07-01): add IUserAccessAuditService and IGraphUserSearchService interfaces

- IUserAccessAuditService.AuditUsersAsync: scan sites and filter by user logins
- IGraphUserSearchService.SearchUsersAsync: Graph API people-picker autocomplete
- GraphUserResult record: DisplayName, UserPrincipalName, Mail

SharepointToolbox/Services/IGraphUserSearchService.cs

@@ -0,0 +1,27 @@
+namespace SharepointToolbox.Services;
+
+/// <summary>
+/// Searches tenant users via Microsoft Graph API for the people-picker autocomplete.
+/// </summary>
+public interface IGraphUserSearchService
+{
+    /// <summary>
+    /// Searches for users in the tenant whose display name or email matches the query.
+    /// Returns up to <paramref name="maxResults"/> matches.
+    /// </summary>
+    /// <param name="clientId">The Azure AD app client ID for Graph authentication.</param>
+    /// <param name="query">Partial name or email to search for.</param>
+    /// <param name="maxResults">Maximum number of results to return (default 10).</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>List of (DisplayName, Email/UPN) tuples.</returns>
+    Task<IReadOnlyList<GraphUserResult>> SearchUsersAsync(
+        string clientId,
+        string query,
+        int maxResults = 10,
+        CancellationToken ct = default);
+}
+
+/// <summary>
+/// Represents a user returned by the Graph API people search.
+/// </summary>
+public record GraphUserResult(string DisplayName, string UserPrincipalName, string? Mail);